In GIMP, you could easily change the colors of the cake, by either or both of two methods:
Either way, get the image into GIMP, and bump up the magnification so it's easy to work with. Then, either:
A: Select and adjust
1. Use the lasso tool to select the portion of the cake for which you're changing the color
2. Open any of the color adjustment dialogs (e.g., hue/saturation), and play with the sliders.
1. Optionally, select the portion you want to repaint
2. Select the paintbrush tool, and play around with the paint modes. In the "color" paint mode, for example, the brush will change the existing color to the selected one, while leaving black, white, and overall lightness and darkness untouched.
I admit, I'm fairly experienced with GIMP, but still, I'm surprised at how quickly I was able to completely change the color scheme of your sample cake.