Combinations of mathcolor and mathbackground.
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"> <mrow> <mover> <mi>X</mi> <mi>b</mi> </mover> <mo>,</mo> <mover> <mi>X</mi> <mi mathcolor="#FF0000">b</mi> </mover> <mo>,</mo> <mover mathcolor="#FF0000"> <mi>X</mi> <mi>b</mi> </mover> <mo>,</mo> <mover> <mi>X</mi> <mi mathbackground="#80FF80">b</mi> </mover> <mo>,</mo> <mover mathbackground="#80FF80"> <mi>X</mi> <mi>b</mi> </mover> </mrow> </math>
Author: Bruce Miller