Blockland uses RGBA; or Red Green Blue Alpha (or you can use hexidecimal color code but don't listen to whoever the handicap is in this thread that said he uses hex to sound smart).
Each of the letters represents 0-255 color value, for example, a dark, solid red would be 255 0 0 255 (or Red 255, Green 0, Blue 0, and 255 Alpha). Alpha stands for transparency, 255 being solid, 0 being invisible (NEVER MAKE AN INVISIBLE PAINT COLOR). Look at other colorset.txt's to get a general idea of how it works.