Help with yellowing plant! Pictures

[mogirl92]

Hi, I am a 20 year old who love orchids!! I study botany in college and had an internship this last summer with the Missouri botanical garden. I have several orchids and they have all been doing good since I had them. I currently have 17 different orchids of every shape and size. I have noticed in the last couple days that on 2 different orchids that a leaf on each one has been yellowing. Both of these orchids look very healthy besides these one leaves. They both have new growth. One of them is a mature plant and has flowered several times before. This one I checked on the root system and it is very healthy. I make sure not to over water any of my orchids. I was always taught it's better to under water them than over water. Could these leaves be changing due too much light? In my past when this was the issue the leave would get more reddish in color. I attached 2 pictures of each leaf that is yellowing. Any advice or help would be appreciated. Also these 2 orchids are not next to each other. Thank you all in advance.