Unknown red/blue Button

[BRIAN123]

Whats it do? I never noticed two buttons , in the center of the dash air vents. One red, the other blue. I intutivetly understand that red is heat and blue is cold but what is the function? Are the on when depressed or off ?
This is a 1990 500SL. The same one I posted about where the stealership wanted $4600.00 to repair the AC and I posted that the ac started to work great all by it self when I declined the repair. BTW, ac still working great, even better than I thought it could.