This is why I miss the Sims 3 Colour Wheel. I used to spend hours colouring hair and adding highlights and lowlights. I think we definitely need more options in Sims 4. We have sliders for make up that keep me happily occupied trying to get just the right shade of lip gloss so why not add this for hair shades too? I can even customise pet furs and coats but not my Sim's hair.
We do have some pre made highlighted and brightly coloured styles as options but the problem with those is that I like to set different styles for each outfit and they don't have corresponding updos/braids/ponies. So my Sim will have to have the same style for every outfit category. I'd love to be able to set the colours myself and have that be the default colour for all styles. I'd like to be able to add different colours for roots and tips too.