What you'll get would depend on whether your roo has a silver factor gene or not. If not then many of the darker colors will override the white in the offspring. If the silver-factor is there, you will get some sex-links and other various mutt colors. Many of the hen offspring will have white in them. The roos will resemble the hen mothers or have mixed colors in various patterns depending upon what genes the hens were carrying. I personally like to breed mutts because I like the different varieties, but my DH hates them.
A rock roo should have very good laying stock as should several of the hens you have, so you should get good layers. If you are just using the offspring for "laying farm chickens" you also get cross-breed hardiness or cross-breed weaknesses in the offspring, but you can quickly tell which you have and cull for the better of the mutts. Someone with knowledge of expert genetics might be able to give you more specific answers, or you could plug the genetic patterns into a genetics calculator to find out what you are more likely to get.