Flat coloring...
Real coloring, not sure what it called, never attend art school...
What the background should be? Hrrr...
Lava lava lava burn burn burn!!!
Here is the lava tutorial, BTW:
1. Fill the background with black-but a little red in it, or color pick example picture (No.1).
2. Use the TEXTURE_HOSE brush, default in Gimp 2.8.
3. Change the brush mode to ADDITION.
4. Change opacity to 40-50. If you are using mouse, set it lower, about 10-20...
5. Use red but a little black as color, or color pick from example (No.2)...
6. Paint it on the background, do it several times...
7. Change the color to orange, or color pick from the example (No.3)...
8. Paint above result of step no. 6...
9. Ta-da!