Bulbophyllum (plumatum) seedling
 

I got the seedling a few weeks ago, and some leaves were a pale green, and others were dark. Is the paleness a bad sign?

I'm wondering if it's too hot (generally 88-98 during the day), or if they dry out too much, or not enough nutrients in the soil.

Is anyone else familiar with Bulbophyllum's?

When you say "or not enough nutrients in the soil", do you have them in potting soil?

No, sphagnum moss.

These are warm growers.... Are the pale green leaves the newer growths? Is the "pale" green close to yellow or is it more of a bright lighter shade of green?

More of a light green than yellow.

newer leaves are usually a couple shades lighter. How much light were they under? try to keep them shaded

Most likely new growth, but could be a precursor to root problems or too high of light.

Update! All the leaves have darkened and the roots are doing fine.