Hello,
This would require some coding and it is not straightforward to do because background colors for post-its are part of a sprite (an image contains all the predefined background, css rules give the offset for each color).
The source code for iceScrum R6 is located here: https://github.com/icescrum/iceScrum-core/tree/R6%2313 and here: https://github.com/icescrum/iceScrum/tree/R6%2313
Making the change yourself may require more work that it is worth, but the code is here so it is up to you. Please note that in the R7 version which is currently under development (the master branch of these repositories), colors can be chosen in a color-picker and are displayed dynamically (no more predefined image).