Get cilantro root if you can. That is what they use in Thailand for the pastes. It wont effect color much at all and the flavor is pretty strong. Coriander seed (toasted) would be my next choice. Just heat the seeds in a dry pan until you can smell them. Just dont burn them, it only takes a minute or two. Third choice would be just some finely chopped cilantro stems. Save the leaves for adding fresh to other dishes..