How to save the memory when storing color information in Red-Black Trees?

Just modify the BST. For black node, do nothing. And for red node, exchange its left child and right child. In this case, a node can be justified red or black according to if its right child is larger than its left child.

Use the least significant bit of one of the pointers in the node to store the color information. The node pointers should contain even addresses on most platforms. See details here.