Flat Colors are the next step. If you use colors that is. Grey shading is fine too, but if you don't do any shading and just finish with inks then your comic can be very hard to visually read. Don't worry about adding shading to any colors yet.
I've just used a paint bucket tool to color the comic. I suggest if you have recurring characters or objects you should make a separate image with a swatch of each of the colors you use on it. Try not to have any single color stand out too much from the others or it will distract from the over all comic page. For instance if I didn't already have a nice yellow fire lane in the top panel I would not want to give the character in the bottom panel yellow hair.