Could be. I'm not really sure what decides the color of the eggs if its the parents genes or just a gene that certain breeds carry. But I'd have to agree with you that they most likely lay white eggs as well. Sorry I couldn't be of more help with this. Maybe as FP stated its your khaki?
ETA...
Sorry just saw your new post. It could be the Rouen cross. I know rouens have a chance of laying green eggs.
ETA: Swedish also have the chance of laying green eggs. So I'm thinking maybe your Swedish/Rouen cross?