I've never seen red tinted cilantro- perhaps you mean purple? If so, they are close to bolting. As hendi suggested, the leaves would get a finer /lacy texture. When they are in bolt mode, the taste gets somewhat metallic.
To harvest, cut at soil level. If they are not bolting, just use whats needed, and it will grow more much like parsely.
If you have enough cilantro plants , you could let a few go to seed for coriander.