Hello
I was given a dendrobium last week by my parents. I got it in a pot with compost as the potting medium.
I repotted him first chance I got with orchid compost (bark) and the next day watered him.
He was a healthy green when I got him.
And now I can see he's got yellowing on his lower leaves and seems a less vibrant shade of green all together.
His stem also seems to be getting darker.

What do I do?
Was I wrong to repot or is this a disease or stress?

Origamifan, it is a Nobile type Dendrobium and I urge you to look through all of our threads here within the Orchid forums as many of them address orchid growing. Many address Dendrobium questions.
Dendrobium is one of the largest groups within orchids. Most Dendrobiums like to be underpotted, yours for a single pseudobulb is in much to large of a pot. This type of Dendrobium likes a lot of water, it never likes to dry out completely.
Dendrobiums have very slender roots and should be placed into a fine media with small particle size. Your media has individual pieces that are way to big. Because of that, and the big pot, you'll have trouble getting it the water that it needs.
Yours needs a few hours of morning sun then just bright. Fertilize once a month very weakly.
The signs of stress that you see are all related to the water. It gets stressed by a lack of proper water. A single growth like yours should be in a 3" wide pot, no bigger.
Plus it is going to have a very tough time making it as a single growth. When orchids get divided, most experts recommend a division of at least three bulbs, not one!!
Simply summed up, your plant has a future, but it won't be easy. An experienced grower would be successful with a single division around 50% of the time. I have no idea what your chances are. Depends on your experience.

To me it still looks okay so far, leaf color looks okay, roots growing nicely..thankfully it is the lower leaf. If it does not spread, I will just let it be. Try to provide good airflow around it, bright light access, much more than a Phal needs. It is still quite young, so be careful it does not get sunburnt. In time, it may have new basal growth or maybe some keikis anywhere up and down the cane.

Don't know your location, but my Dens during summertime gets frequent watering here. They handle our warm to hot dry weather even our heat waves, as long as they get good airflow around and daily watering here since our area is bone dry.

So analyze your growing area and adjust the watering as your growing situation needs it.