-bpp

From Quake Wiki

-bpp allows you to specify a color depth in glquake and derivated engines.

commandline parameter effect
-bpp 16 16 bit color depth
-bpp 32 32 bit color depth (recommended)

32 bit color depth uses slightly more video memory but nowadays that is negligible. If you are using an engine with fancy smoke effects those will hugely benefit from 32 bit color depth.