No problems grazing there. Horses will graze a pasture by eating the grasses from most palatable to least, moving back and forth. Unless they're starving, they won't eat anything that is bad for them...even in a well grazed pasture there will always be places they don't eat.
I wouldn't worry about colic. If anything, you'll want to start the grazing at a couple hours and let them get used to it and minimuze potential hoof problems (sounds like it is beyond the real lush stage and probably getting stemmy, though)