Not sure how to go about creating a color picking GUI?

Just trying to plan on how to make a color picking GUI and I was thinking maybe doing it by brick colors? I'm not really sure what's the most efficient way to make this?

There are tons of ways to make a Color Picking GUI. As long as you're using your brain, you should be able to make a Color Picker in multiple ways without needing the worry about it being taxing to the game.

I would suggest that when you make your Buttons, make sure the color is very close to the color the part will receive, a way to do this would be doing the given code:

GUI.BackgroundColor3 = game.Workspace.Part.BrickColor.Color

BrickColor.Color returns a Color3 associated with the given BrickColor, allowing you to make a GUI the exact color for a player to see. Now that a player can properly view what color they'll select, you could just name the GUI the BrickColor and set whichever part is associated to have that color.