How do you express your sims' personality visually? (decor, fashion, etc.)

I love, love, LOVE CAS, it truly is it's best in 4 (minus one notable exception...) and between walkstyles, accessories, skin details and the like, I can spend literal hours* refining a sim's look to represent a sim's personality, even if some of my own choices strike me as a bit obvious (like, what alien would actually wear an alien t-shirt? ...unless they intend to throw everyone off!). In particular, I like assigning walkstyles based on personality, so I have a bunch of erratic and goofball sims with the goofy walk, and snobby and materialist sims with the snooty walk. I ADORE seeing my simmies out and about, strutting their unique walks (not so unique after making so many sims, but, what can you do? *cough*AddMoreWalkstylesAlreadyMaxis*cough*) and it helps me remember who's who ("Oh right, he's my comedian! That's why he walks like that...").

I also really like playing against type. I have one sim whose hotheaded, mean and evil... and is the cutest pink-clad sim around, down to her perky walk and high-pitched voice. There's also my big bad werewolf (totally not just a rebranded vampire, nope :|) guy who looks mean and has a tough walk, but is really just a big sweetie and wants to make friends.

*I've had it take literal weeks to create certain sims because I want so desperately for every part of their look to come together to really hint to their personality. I might have a problem...

Yep, that was me and I just wanted to illustrate what I was talking about :)
It's not really about reflecting personality, it's more about my visual aesthetics (or Obsessive-Compulsive Disorder LOL).
Anyway, that's how it looks for some of my households, both played and premade.
https://i.imgur.com/gYRCrK9.jpg

Sometime the color is just something that goes well with the appearence.
Sometime, like the family in my profile, it reflects their theme, which is nature, gardening, flowers, herbalism = a lot of green.
https://i.imgur.com/pRJjusd.jpg
That's how it goes for outfits and interiors, if anyone is interested:
https://i.imgur.com/MHpsEIO.jpg https://i.imgur.com/71Px19r.jpg

I've done a few colour coordinated individual sims, even with their own colour coordinated rooms. it's rather interesting.

Tobias Lancaster, one my favorite sims, he wears white long baggy shorts, long white socks with white shoes with a white sweater and tee underneath and then a backwards white cap. He prefers plain white colours in his room and workout area. When he manages to woo Summer Holiday and they live together, she looks great in white as well with a few minor adjustments to her appearance (no changes to her face or body though).

Once i've cleared this new member limitation, i can provide screenshots.

I play my sim with autonomy for a while and see what kind of things that sim does, wants etc. Then I give them the fitting makeover because I kind of have a sense of what personality they have then. I go for colors and outfits that just look perfect on them and fits their personality. I like to send them to meet people and watch how they interact with others. I also keep my eye on their moods, like some sims can do things that get them energized more frequently (without having the trait for it) so that gives me the idea this sim is into sports for example. later when sim ages up I give them the matching trait and sporty outfits. So I kind of let them decide a little on their own and once I have a picture of what they like I take over control. I also take into account where I run across them when I play rotational. Like one sim I would always see at the lounge playing guitar, another one is always at the gym, the other one always at a party etc.

When I decide to build a house or change the interior of their house I often take the colors of the parent/s for main rooms. For the kids rooms I take the colors and hobbies/skills from whoever sim lives in that room. When my sim has a baby I will give them the colors from the parents until I figured out what works better for the child. I do the same with their baby/toddler/kids rooms.
The parenthood pack works in my favor here when the kids ask for advice, I know it's random but I find it really matches their personality in a way. Or these little messages that pop up when the kids are in school. Like principal caught teenage son skipping class because he was consoling a friend. I often get this message for this son and it fits, he would do anything for his best friend and he's been through rough times lately so it's understandable (lmao the line between make believe and believe is getting really slim here xD).

I only do this for sims I play, I don't give townies a makeover unless they marry into my sims families somehow.

Hmm, my Sims and their homes reflect just my own preferences. So, more or less same style always; laidback, casual, simplistic and natural colors. I'm a little set in my ways, probably in part due to my age, but I also have eyes that don't appreciate vibrant colors, and a brain that doesn't like too much clutter.

My choices in CAS always reflect the idea I have for a Sim's interests and personality. My legacy Sims are stuck with the choices I made several generations ago for the house, but when I decorate the toddler-kid-teen bedroom, I like to use colours and styles that reflect my intentions for the kid. (Which is why having just one single bed with a decent sleep rating is so annoying - the Cordelia bed is just not suitable for my geeky tech teens.)

I don't remember if I was the one who said it, but I do usually associate certain sims with certain colours, which are reflected in their clothes and bedrooms, but not necessarily the rest of their house (especially if they don't live alone).

Other than that, style is also important. A family-loving religious grandma obviously isn't going to have blue hair and revealing outfits in her room that looks like a gothic tomb, but rather wear comfortable pullovers and long skirts with floral patterns, her room being filled with old-fashioned wooden furniture and photos of her children and grandchildren. Her twin sister, on the other hand, might be her polar opposite, a femme fatale even as an elder, characterised by leather pants and makeup to look younger, along with a high Charisma skill.

Then there's hobbies. Sims who live alone or with like-minded people (like a group of musicians) will usually have their hobbies displayed in one way or another all over their house, whereas for family homes or apartments shared by people with very different interests, their bedrooms usually consist of the usual stuff (bed, mirror, wardrobe) plus at least one item representing something they like to do in their free time.

I colour co-ordinate as well. Roddy, the firstborn of my current generation of Furystrykars, loves light blue and pink, and his voice is set to sweet, with the pitch slider going to the left. He also wears makeup and dresses sometimes, something I did with Drake from my Sims 3 game.

I mostly express it through their style, the interior design of their house/room and what skills they have. I also have a tendency to give them a "favourite colour" in a way, by having most of their clothes and bedroom being in said colour. Usually I do this on big households so I can easily see who's who when they walk around :D .

Back in Sims3 when we first could select a 'favorite color' for our Sims, I went all out decorating my Sims' room (if they were still kids) or their houses making sure to use their favorite color, thinking this would give them a positive reaction. It had no effect whatsoever. Now, I just decorate their houses in whatever fashion design I think would suit them. My Rock Star had his house built, for instance, and it took me hours to decorate it in a manner befitting his Global Superstar I'm a wealthy Sim status. LOL

His dining room:
https://i.imgur.com/ZV68y7z.jpg

The view of his formal living room from the foyer:
https://i.imgur.com/LuKXhlO.jpg

View from the opposite end:
https://i.imgur.com/wPG2oTg.jpg

A glimpse of the Master Suite:
https://i.imgur.com/XeXK7sx.jpg

the Foyer:
https://i.imgur.com/Ua3j0ZP.jpg

The Butler's Pantry:
https://i.imgur.com/yxrT7ET.jpg

The Mansion Itself
https://i.imgur.com/i4oXsb7.jpg