Bring up the console and type the following
dimamount - changes the translucency of the color tint (0.0 - none, 1.0-fully opaque)
dimcolor - changes the color of the tint. The input is in hex ("FF FF FF" is white) or you can enter one of the predefined colour names located in Gzdoom.pk3, in the file
x11r6rgb.txt. The latter is easier imo, but personally I stick to yellow
