The spike started yellowing on mine as well, but stopped right above the keiki. Normally if the keiki is still pumping nutrients from the mother plant, it should stay green starting at the keiki and below. Keep an eye on it just in case, you might have to cut it off if the yellowing progresses down the entire stem.
There are good threads about keikis and coconut milk and other. I'll try to find them, other wise I'm sure Dorothy can find the info for you quicker than it takes to say 'keiki'!!