Typically how I work is to have the lineart on a layer above any of the colors. If you have a png then the image will already be transparent, if it is a jpeg then you just change the layer to multiply and the white parts of the image will act as if they are transparent.
It is a much faster way to work than using the magic wand to select everything, especially if you are working with a low resolution image. If you are working with photoshop then you can ctrl-click the layer you have your color on and it will only select what is on that layer so you don't have to worry about painting outside, alternatively you can choose preserve transparent pixels on the layer and it will do the same thing.
working with the magic wand makes it ugly and all pixelly kind of. I don't know much about your style and the technique that is involved with it since my style is very different but from my own experience I would advise you to put the lineart in a separate layer over the coloring layer and start coloring with the brush like that and erase if you cross the lines, I think it would be much easier and you wouldn't have the pixelly effect and the annoying white lines you get with the magic wand. (pixelation might also be caused by file resolution btw, never work with less than 1000 px X 1000 px
Of course it took a long time, it's your first digital piece. If it was super awesome fast and easy we'd all be *sakimichan
My advice would be to experiment with lots of different methods. There are heaps of different ways to colour as you can tell from just reading this thread. I personally don't magic wand or mask anything and just paint with the brush. But then again my lines are thicker than yours. So test out a lot of different ways to find what works best for you. You'll get quicker as you practice.
That's my suspicion. I'm learning how to mix colors onscreen and it does take quite a bit longer at first than with paint. But, OTOH, I spend less time recreating something that I've screwed up when working digitally. Also, once I've got the colors I want to use, recreating them is extremely fast whereas with oil I would have to regularly remix what I needed if I didn't prepare enough for the whole painting.
I suspect that as one practices more and more, the time it takes to mix the colors gets less and less to a point.
Yep, but for practical reasons it's nice to keep the major colors on a virtual palette so that it's easier to grab. Depending upon the style of painting, it can be a bit difficult to click the right pixel sometimes.