RED LEAVES ON BRASSICA PIC

Here we go -this is the strange happening with my brassicas....it does look pretty but it really shouldn't be happening !!!!
Its strange as i only gave them some fish blood + bone food a few days ago -and Im sure the other plants are starting to lose the red tinge -although it could be my imagination lol -so now I have a picture I will be able to compare and see if lack of phosporus was the problem ..

Brassica leaves turning purple!!!!

I have decided to do a trouble shooting section as I have just had my own problem and have tracked down the possible cause and solution -so thought it would be a good idea to keep it all together...

Anyway -the title says it all lol - I noticed a while ago that many of my brassicas -particularly the brussels and cauliflowers had started to get a purple/red hue to their leaves. I left them as it didn't look like they were diseased or dying -but as more have started doing it I have trundled through the internet and found that the probable cause is a lack of phosphorus -which is needed for photosynthesis (all sounds very technical lol) .... now I don't want to add too much in case that isn't the problem so I have use a bit of fish blood and bone fertilizer I happen to have in -pinkies crossed it helps

i will try and get a picture later so you can see what I mean and I can compare for any improvement ...