If the maximum practical speed of interstellar colonization is significantly slower than the speed of light (and assuming mass/energy as we know them remain scarce resources, e.g. advanced civilizations don't Sublime into hyperspace or whatever), then we would be able to observe advanced civilizations in our past light cone whose colonization wave hasn't yet reached us.

Nitpick: If the civilization is spreading by SETI attack, observing them could be the first stage of being colonized by them. But I think the discussion may be drifting off-point here. (Edited for spelling.)