#1 - Why are the squares already painted??
The squares already have a base color scheme to make sure that the mosaic pieces will all work together. If everyone follows the color scheme and design in their piece it will make up the overall design when all the squares are put back together.
If everyone just painted whatever they wanted, the mural would be total chaos!
Go to: http://muralmosaic.com// to see examples.
#2 - Can I totally change the colors that are on the square I chose?
No. You need to follow the general color scheme of the square. If it was overall a light blue, you can use pastel colors to paint your square but it needs to be predominately blues. Think monochromatic color scheme with hints of others colors.
3# - Do I need to keep the main design elements in my square?
Yes. Those design elements are what will keep the overall design in tact. The edges can be softened but not totally deleted.