It also dictates what map you're going to see if you plug a gba in the second slot.
White: normal (blue map): shows the layout/doors
Red: Monster (red map): monsters appear as haircrosses, boss appears as a red haircross
Blue: Treasure (green map): Shows the chests and the boss as little boxes and a haircross
Green: Scout (yellow screen): shows the information about the last monster hit (HP, weakpoints, attacks...)