I'll say it is up to the story you want to tell, how much have you experimented with colors, effects and layers and what kind of drawing you want to color.
The more details a panel will have, the harder/more time it will take to color it.
Let's say, in my case, TMBD has a lot stuff regarding sunsets, the sky, the atmosphere, I want to have those really clear for the reader, when it is dark, when it is a sunset, when there's fire and thus a strong light, so I do it in color, but coloring takes like 2/3 of the time when making the comic.
However, having to color the comic has made me found some moments where I don't kwow what colors use for some panels, early one I had trouble with how to set the atmosphere for a "shocking" scene where a character gets shoot, and up to this day, I still don't like the colors I used for those panels:

But then again, some can express more in black/white or grey-scale, you know, less can be more.
Bleach, though later badly received for having way too empty panels, that's something that can work too as it leaves the rest for the reader to imagine, and if done well, can be a good thing for both saving resources and expressing better stuff that if colored, wouldn't look as nicely.
I could also talk about REDUX by The Letter M, it uses grey-scale and some colors for the characters, one uses red, other blue, other green, another a scarlet-ish red, and it looks well and translates great for the story's atmosphere and even its development, as the colors are relevant for the powers of the characters.