I tried GUI.Color but it doesnt work
Umm, you simply create a custom GUISkin and add it to the element declaration.
However if your ARE NOT making editor extensions, I highly suggest you do not use GUILayout and instead use the new UnityEngine.UI.
Which is even easier to change the look of.
To make a new GUISkin, simply right click on your file-system in Unity, and go to Create > GUISkin. In your MonoScript you can add a public GUISkin mySkinand drag’n’drop the GUISkin into the script.
EDIT: If you want multiple colors and such throughout your GUI, I’d suggest doing the GUISkin in code, even if you don’t though, you can change the colors and whatnot before each new GUI element is declared.