2dcol is a definition of a color to be used in the 2D mode of Mapster32, in conjunction with spritecol that works similarly to tint and definetint. <colornum> is a unique identifier, similar to defines and gamevars. <red>, <green>, and <blue> are the RGB color values that are mixed together to get your custom color.
Certain low values are already hard-coded, and redefining them will change colors of the default layout. If you know the correct value, you can replace the default colors used for all the sprites. You truly have the power to customize Mapster32. Lime green 2D mode background, anyone?
See also spritecol.