For example the colour picker in paint.net, or something. I'd want the player to be able to choose from a colour wheel and then it colours their house or something similar.
Hey Twenty!
So, I've never actually done this before myself, but having experimented with how the color picker in Roblox works, here's how I'd tackle it:
Note: You can change a part's Hue, Saturation, and Value with Color3.fromHSV(hue, sat, val).
The max Hue in Roblox is 359 and the saturation's max is 255. Therefore, it might be in your best interest to find an image around that size (to make the math a bit easier). The value slider's width doesn't matter, but a height of 255 is again going to make it easier on you.
I hope that helps!