Leaves are turning purple?

[Watige]

The leaves on my blue dream are turning purple and I'm not sure why?

 

[Chris Scorpio]

Could be a phosphorus deficiency

What medium?
Any nutes?

 

[Watige]

Could be a phosphorus deficiency

What medium?
Any nutes?

I have included a pic of the soil I use. I have given them Fox Farms Big Bloom and Grow Big liquid nutrients. I've got 3 plants with this problem.