Red-Black tree storing color without additional memory requirements

[simendsjo]

Wikipedia states that the color bit can be stored without taking 
additional space: "In many cases the additional bit of 
information can be stored at no additional memory cost."

Looking at the Phobos implementation, it stores it as a regular 
byte: 
https://github.com/D-Programming-Language/phobos/blob/master/std/container.d#L4945

The only way I can see it storing a bit without taking additional 
space is by storing the color in the pointer to a node instead of 
in the node by using the tagged pointer trick: 
http://en.wikipedia.org/wiki/Pointer_tagging

But I would think this trick would break the GC, as well as 
making code less portable.

So.. Is the RBTree article a bit off here, or are there other 
techniques to reduce memory overhead?