I'm going to very strongly agree with this suggestion, as I feel its very important, but for a slightly different reason.
Many modpacks add quite a variety of liquids - but when "diving" into them, the fog color is either water or lava.
Ability to override the color would increase the immersion of these mods significantly.
Ability to control the depth of fog present would be quite useful as well, for custom liquids as well as for reasons described here (custom equipment, potion effects, and the like).
TL;DR: +1.