Even using a program like Photoshop, coloring a comic is a pretty time-consuming process. One of the ways that I speed up the job is to "paint" one color at a time to every place that needs it on the page. I usually start with the background color and/or the most prevalent color and move toward the lesser-used colors. I generally try to "build up" one color on top of another. That allows me to be a bit sloppy and paint outside the lines (which also saves time). For example, I can lay down the light blue background without worrying about coloring over the characters, because I know that I will go back and color over anyplace where I colored outside the lines.
In this comic, the steps were:
Step 1: background blue
Step 2: wasp red
Step 3: wasp blue/purple
Step 4: carol red (a different red)
You can see that Wasp's face is still blue at this stage. And some of the Carol red spills over onto her neck and hair. I'm not worried, because I know that I will cover them both up shortly.
Even putting down color in this manner still takes time. I'm always finding little areas that I've missed, so I have to go back to an earlier color. And you have to get progressively more careful (which is especially hard the longer you work on something, because you get tired and ready to be done!).
I left the colors pretty flat. I did do a shadow layer to add some depth, but otherwise there are no fancy tricks here (like using gradation or dodge/burn). That is partly because I don't think I could have spent any more time on this and partly because I felt like it looked decent enough for a silly joke comic. (On the downside, I feel like the blue background really highlights the fact that there is no actual background.)
I didn't really track my time, but I the whole coloring process easily took six hours (but I wouldn't be surprised if it were closer to ten hours).