It depends. What effect are you trying to achieve?
If you want the daytime to be bright, set the surface to pure white, and it will not be darkened at all.
If you want the daytime to be yellow tinted, set the color to fully bright yellow. (255,255,0,255)
if you want the scene to turn blue, set the color to (0,0,255,255).
If you want the scene to be slightly darker, with a little bit of a blue tint, set it to something like (192,192,255,255)
If you want it really dark and slightly blue, set it to (64,64,128,255)
The reason to use modulate blending is to darken the scene without making it look smokey and desaturated, like what happens when you just use a regular semitransparent solid color on top of it.
That is, if you used a dark color, with blend mode Blend, at 50% transparent, the blacks would lighten and the lights would darken.
With color_modulate the darks stay dark, but the lights get darker too.
With additive, the lights stay light, and the darks get lighter, but colors stay bright, instead of being washed out, like the same that happens in the previous example.
Color choice is really important with blend modes.
Additive blend is really good for things like fire and light beams and mirages, modulate is better for things like shadows and color tinting.
Light colored mods are best for slight effects, while dark colored mods will have more severe effects. Pure white mods will be invisible. (multiplying 1 * whatever)
Dark colored adds will be better for slight effects, like a dim glow from a candle, while brighter adds will more sharply glow (until they reach pure white). Pure black adds are completely invisible (adding 0 + whatever)
