Color change
 

It looks like I have 2 Catts in the same container- one of them is developing a purple hue in its leaves. The other had a leaf yellow and dry out- it now has a second leaf starting to possibly do the same.

It/they is/are currently living on a heating mat and a thermometer/thermostat keeps the root zone around 72. It is getting 14 hours from a grow light. I water it once to twice per week depending on how dry the media gets and fertilize it weakly/weekly with MSU. It also gets Cal/mag once every 4-6 weeks.

Is purple okay? Should I separate the 2 Catts into their own pots? Other thoughts? Pics attached.

Thank you!

Some Catts. especially those with dark flowers, will show red color in the leaves especially in high light. It is very possible that you have 2 plants in there, but I would suggest to wait on any repotting until you see new roots. You may need to increase the watering, the heat mat will make it tend to dry out faster.

You should investigate this term: Anthocyanin

It's a pigment that in plants causes a reaction to light intensity, the same reaction as we, humans, experience and know as "tanning".
In case of plants with high light requirements, this is usually a sign that the light levels are adequate.

14 hours from a grow light is probably a good amount of light, so the "purpling" is probably what rbarata mentioned, anthocyanin. It's not in itself a bad thing, but your plant will continue to make it's leaves purple-y under high light.

Usually, this is due to the intensity of the light when using grow lights, so maybe get the plant a couple of inches away from the light, or move the light up a couple of inches.

If it keeps happening after you've moved the plant or the light, try 12 hours of light exposure.

I would also do what Roberta says, just in case.

Almost no epiphytic orchids in habitat get 14 hours of light. 10-11 hours in winter is enough. Seedlings do better in lower light levels, with no purpling. Those need more frequent watering - they're shriveled.

I have heat mats under my Catts, Dens and Oncs over winter.
The heat rises up through the pot and takes the moisture with it.
I water those on the mats at almost summer frequency, every 3 days.

I decide when to wster by looking at the plants. Those are shriveled and need more water.

Watering on a set schedule is convenient for the grower but doesn’t always work for the plants. Having a set schedule for *checking* if plants need water is great.

I don’t grow these, but your plants are displaying the classic signs of stress (slight shriveling, purple coloring). Like the others have said, reduce the light duration a bit and try watering when the plants need it, not by a schedule. You can also try placing a plastic barrier on the top of the medium to slow evaporation.

I give my phals. very high light and many develop reddish/purple coloration, which is fine, but they’re not young plants. Babies need a gentler touch.