yellowing on my dendrobium's leaves

[Origamifan]

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.
See attached photos

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