I can't really tell what's happening on the left side of the picture to be honest. Is that his hand or somebody else's? The coloring doesn't really look good at the moment. The colors goes outside of the line and sometimes don't even fill up the whole space and it's somewhat flat too since there isn't any lighting and shading to give it a sense of depth. Did you use MSPaint to color this? It's really not a good idea to use MSPaint to digitally color. It's best saved for Photoshop and GIMP where you can make masks and have a wider range of tools to use.
A mask essentially allows you to say what parts of an image you want to work with, and what you don't. Would you mind showing what the original art was?
This isn't the best explanation but this will have to do. A mask is basically a custom made shape on top of a layer. Instead of going through the process of carefully painting inside of the lines. You could instead use the pen tool to make a shape of whatever you need to paint(such as the hair or head) and then fill the shape with the necessary color after you're done. EDIT: Or what Mixt said xD
This is an okay attempt I would say. If you used layers and kept the lineart at the top, you could have done multiple layers with different colors and then colored like that.
That does not look like lineart, that looks like a straight up scan from a manga. Like the other comments it looks like you used the fill tool on mspaint, from what I can see as how blue is spilled out of his hair.
I think you may just be approaching manga coloring wrong. =] It can be a really long process, but it's very rewarding at the end. The issues others pointed out are true; it's not enough to just fill the areas and put it above the scan. The fast way is, if you have a really good scan, is to put it on a layer on Multiply, then do your coloring underneath it, on separate layers. What I used to do, though, is completely redo the lineart to make it cleaner and get rid of all the unnecessary background detail, then doing my coloring underneath that. It takes a lot of time and the pen/path tool can be frustrating if you haven't used it before, but it produces much better results (imo).