The Figures
Left: Purple hair purple suit variant Right: Black hair blue suit variant
Here we have the two Apocalypse BAF wave Psylocke variants. The black hair version was in the earlier release, and is harder to find. I had to pay slightly more than retail price to get that figure.
They are basically the same exact figure with the same accessories. The only difference is the colour.
Differences

Left: Purple hair purple suit variant Right: Black hair blue suit variant
You can see the difference in colour; purple hair and purple suit vs black hair and blue suit.
Left: Purple hair purple suit variant Right: Black hair blue suit variant
Clear hair colour contrast in this pic above.
TL;DR
They are the same figure with the same accessories. The only difference is the colour of the hair and suit.
My Verdict
The black hair is harder to find to me. It’s basically just a preference of colour. Both are great to me, I’m keeping them!