something like /colortest while ingame, or you could use hex codes (<color:######>)
the first 2 digits being red, 2nd 2 being green, last 2 being blue
the digits can be 0-f, 0 being dark, f being bright
<color:ffffff>
<color:000000>
<color:ff0000>
<color:00ff00>
<color:0000ff>
<color:ffff00>