Purple leaves: Cold damage, Insect stress, too much light, magnesium or phosphorous

[Shadeflower]

wow these have been some fantastic answers, thank you for your observations.
technical_support yes I agree thris are the devil. I mean at one point I counted over 30 infected plants in 2 weeks.
And they keep coming back. It's like they are super mario with 3 extra lives. You wonder how. Even after soaking the orchids for an hour. They still are able to come back.
I have just stuck to my routine and I have been writing down which ones I treat. That way I know which ones should theoretically be clear and which aren't. They seem to operate on a monthly basis. Ie you kill them all (some eggs must survive) and a month later you spot them again.

And then if you do not get rid of them asap they start hopping onto about 5 plants next to them and hide there for a month to repeat it all over.

So I still will stick to believing that thrips weakens them so much. They nibble on the roots mainly so the orchid is constantly wasting energy regrowing root tips. What does roots growing need, phosphorous, so with thrips they get more depleted than without thrips. Or it's unrelated but I have seen too much positive effect from adding a bit of phosphorous.

tedro, good to see you do use a bloom booster but like I warned you they are really powerful. If I had yours I would be adding 0.1 to 1 ml to 10 liters. That is to your regular feed, I personally wouldn't use the bloom booster by itself...

the aim imo over a year is to achieve a 1-1-1 ratio.

So with 11-35-15 and 13-3-15 one can see that that can be achieved easily using the 11-35-15 for 3 months and the 13-3-15 for 9 months of the year or both together in a 3 to 1 ratio (3 part grow to 1 part bloom) and overall that would work out the same as using a 1-1-1 feed all year.

I also discovered that root boosters are the same product as bloom boosters. Basically phosphorous and potassium in a higher ratio to nitrogen promoted rooting and flowering.
Nitrogen in a hgiher ratio encourages growth.

I think the thrips have made me paranoid about feeding them when it is important but maybe not as important as I am thinking.

Like I am worried that feeding more phosphorous without adding potassium might have started a potassium problem next. IF you increase the one, then the ratio to the other changes and if they were 4:1 before then changing the ratio changes the availability to the orchid.

Maybe that is why phosphorous is generally sold with potassium?

I'm glad I got some good feedback on this. I know I'm not the only one who would love to understand nutrients better but it is challenging. I feel I am 1/20th there lol.