Blue, red, yellow and green are the distinctive Google colors. Those same four colors have been Microsoft's colors ever since Windows was launched. Why is Google using the exact same four colors that have been Microsoft's for long? Was it a coincidence? Of course not.
My guess is that grouping those four colors represents a product made to be used by everyone. Windows was launched to be the easy operating system for everyone, not for an exclusive group. Google was launched to be the same, a service that is easy to use by everyone, not an exclusive group. I don't know why those four colors in particular convey this meaning, but sure marketing research experts are the ones that have the answer.