Hi,
1. Try to mix the colors more. Jabba is fine, but on the back of the Summoner a mix of 3:1, 1:1 and 1:3 of the colors in the middle would help.
2. As Sigmar said, try glazing. Dilute the paint, remove the excess paint from the brush on a paper towel, so the brush remains damp (not wet!) and paint over the colors. Darkening is easier than lightening, especially as the lighter colors tend to be chalky.
3. Try stippling! Spend 1-2 nights on trying stippling. Use it on some organic part, like cloth or leather. Easy, quite fast, gives a nice texture, and the best painters (e.g. Bailey03) are using it. Just spread some random dots over the border, more closer to the border, less farther away, and the dots you do not like can be split in two or covered with the other colors.
Edit: +1: A nice Tzeentch logo would help on the back, too. As you did not use much color on the black, it should be easy to fix any mistakes you make. As you can paint the text in the codex, you should be able to paint a logo, too.