I was tempted to vote for the color wheel but I'd really love to have an RGB, Hex, or CMYK color tool. With those tools you can get very specific with the colors and shades that you want. It's less fiddly than trying to get the cursor on the color wheel on exactly the same color time after time, object after object. With those tools once you google the codes you can repeat that precise shade every single time. It may seem daunting but those code systems are extremely easy to use and you get precise, repeatable results that are far harder replicate with the color wheel.
