Freezing Alone

I'm able to pull the 10+ hit combo off with Muramasa (I get 21-22 hits once all is said and done) but the trophy refuses to pop. I'm playing against the CPU and I'm positive that I'm doing the entire combo correctly. The only thing I can think of is that the combo text goes from red to grey halfway through (around the 7-hit mark). What does that mean exactly and could it be preventing me from getting the trophy? I'm playing on the NA version btw.