There is something about the color palette that needs some energy or something.
Try experimenting with these:
http://www.pictaculous.com/http://www.palettefx.com/Organize or look for images that reflect the feeling/energy you want to capture and feed those images into the above links. Then use those colors or suggested colors. There is a lot of material out there for the TMNT old and new and the studios spend a lot of money on advertising and getting the right images for promotion. Why not take advantage of all that work done.
Looks like you are using new renderings of the turtles and the older graphic from way back when, you might want to see if you can somehow link these together, unify. (Color/Eroding/changing) What about place? Let that inform your work. If they are in a sewer... make it so. If its in your living room... well... make it so.
I used the image below, just to give you an example right away. Pictaculous boils it down to 5 colors, a good starting place.
You might want to try adding some depth to the images. For example: Blurring/focus/shadow/obstructions/brightness/contrast the stripes to set them back. Think of each part of the image in layers. Background, mid-ground, foreground, etc. etc. What is most important?
Hope this helps you get it done.
Cheers!