Somewhat of this can be achieved my means of applying Tinting to the map --and the minis inside it if you want as well. The Tint option is available at the Token Edit window.
I did the following test for recreating part of Seedle's example:
(. . .) a really powerful evil is being awakened via a ritual and the area gets a red tinge to it as the evil starts awakening.
Here is my usual testing map with no tinting:
And here is the same map & tokens with an evil, ominous red tone:
What do you think about this approach?
Note 1: the Background & Maps layer doesn't allow to edit the image for adding tinting, so you need to move it to the GM info Layer, apply tinting, and move the image back to the Background & Maps layer; this isn't happening for tokens, as they can be tinted in their own Objects & Token layer.
Note 2: in this way, only the contents of a Roll20 page canvas can change color for enhancing the mood and narrative purposes. This doesn't change the color of the Roll20 UI, which as I said above, would work wonders for me.