i think red g's start at +5.7 gs, but if im wrong then im only about a tenth off. And I don't think yellow g's are considered as a g-spike, but many people do consider them as one.
yellow and red g's are in-themselves not G-spikes. A G-spike is when the g levels sudenly go from low to high or high to low. For example, if you where going down a hill and where pulling -1g, then when you got to the bottom, in about a tenth of a second you go to 3g's, that would be considered a G-spike. yello g's are just high g's, and red ones are higher. That's all.