>>2669754>>>>/ic/ should have some resources to learn from. I'm not going to go into depth because it'd take up a whole thread that isn't /y/ related. There's just two things I want to point out.
1) Get a program with access to layer functions. GIMP and Paint.net
are free, poor man's Photoshop. I think FireAlpaca does as well. I personally use Paint Tool Sai.
Layers let you work on an area without affecting other areas. Think of it like painting on multiple transparent sheets. If you paint something on one sheet, it doesn't change the sheets below it.
When you get line art from somewhere, by default the program treats it as though it's opaque - you can't see through it, like paper. Setting the layer mode to Multiply, highlighted in the pic, makes the program treat it like a transparency. You can then make layers beneath it to color on. If, in your picture, you had a Multiply line art layer, you could sloppily paint everything beneath it green to easily fill in the white holes you have in that text without fear of it fucking up the text.
2) Your colors are really saturated. Don't be afraid to pull colors from somewhere else. On the left side of my screen grab, that section with all the colors? Each of those quadrants are from a different picture whose color scheme I liked and stole.
Find a picture that you like with similar colors to what you need. Copy->paste it into a layer to keep it handy. Use the eye dropper tool to grab the colors you need as you work. As you can see in pic related, I've taken some screen shots from the source material that I'm going to be using as my pre-shading base colors. They look stark at this stage, but as I put shading down they should become more unified. Marco Bucci on Youtube has a series, "Ten Minutes to Better Painting" that, I found, was a very approachable introduction to color theory.