jmc

There are 3 ways for John to decide which of the red, green, and blue marbles to choose. After he has chosen one of them, he must choose 3 marbles from the other 9. There are $\left(\genfrac{}{}{0ex}{}{9}{3}\right)=84$ ways for him to do this. The total number of valid ways for John to choose four marbles is $3\cdot 84=\boxed{252}$.