I can see the purple tint, not much but it's there. A run off pH of 5.8 is too low so it does look like it's lockout. That surprises me a bit since Pro Mix is very consistent and usually 6.8 - 7 right out of the bag, so if anything, usually needs to be lowered a bit. A foliar feed should help. I don't think it's so low where anything drastic is needed, just up the pH of your feed to around 7 and check your run off again.
At 26 days old they should still be on veg nutes. Have you amended the Pro Mix with anything? Also, when did you start feeding them veg nutes?