Make sure the hair is parented to a figure before you apply a MAT pose. MATs don't work on prop hair unless it's parented to a figure.

If that's not it, it could be the old memory issue. It was a bug in P5, and may be still be around in P6. Sometimes, Poser keeps a texture in memory, even after you've changed it. If the hair doesn't change color even when you apply a different texture manually, this is the problem. The solution is to save your work, shut down Poser, and restart it again.

And make you've installed the SR1 patch for P6.