What do you mean by "colony"? Generally, when I think of colony, I think of a large amound of zoas already attached to a medium sized rock.
If you are talking about a frag that is loose from the rock (I suppose you could have a whole colony loose from the rock though) I would actually reccomend picking up some super-glue gel. That stuff is amazing. I cleaned up my prop tank in 10 minutes and everything is great and healthy. Maybe its just my luck, but even in a lower flow area, we had no luck getting zoas that didnt already have a small rock attached to them to attach to a new rock.
If you are talking about a colony spreading to another rock, that depends on a lot of things. Light, flow, etc.