Decompile the client in rabcdasm, use AstroGrep and set the search folder in AstroGrep to the client-1 folder in your rabcdasm folder, take a screenshot of the color you want to change on the client, find the hex code (use paint.net there is a tool for that), convert it to decimal, search for that decimal in AstroGrep, replace with the new decimal color and save and then run 02 bat to recompile.
If you want to change the grey side bar, like the background color behind the inventory, equips, health/mana bar, etc. well then that decimal color will be found in like 40 files, change them ALL because
1) itll change the color of the register/login box, other panels, etc. and best gui is when those match with the ingame gui
2) idk the exact file JUST the side bar is in