If you can change the colour at all, then you'd need to draw it in blue, then slightly smaller and offset in red.
That doesn't sound like a brilliant solution to me, 'cos you'd probably need 2 fonts at different sizes to do that.
Combine that with the fact that d3dxfont is not a fast API at all.
...later...
Actually, I was looking for a Text3D sample that used to be with DirectX. It implemented a class called CD3DFont which pre-rendered a font onto a texture, then let you draw it at any size/position/colour/orientation and it was really quick. Looks like that sample has been dropped in favour of d3dxfont sample, and I wonder why.
I've used CD3DFont before and it was cool, but it needs big textures to look good, and right now I can't find an archive of the sample - it's part of older DirectX9 SDK samples.
Jim