How Do I Get Grey Paint To Turn Purple?

I would first try adding a true blue or an ultra blue to see the color change.  Just take a small amount of the grey and using a tiny toss-out container mix together to see the color.  Then add a smaller amount of red (watch out as red is very strong) and this should give you purple.  Experiment with the amounts of the blue and red added to the grey until you get the desired color of purple.  Make a note of the proportions used.  You may have to use another container for the final product as you'll be adding paint amounts to the original.