Hey sorry for the delayed reply, you just have to put another colour in the game data on the line after the pallete you want to add them to and then put COL <index of your color> on the line after the tile, sprite or item you want to colour.
Colours are indexed from 0 so if you added a 4th colour and wanted to apply it to a tile you would put COL 3 on the line directly after the last bit of data for the tile.