What is the fertility program looking like?

It could be roots.  Root rots could impact nutrient uptake, which would explain the yellow color you suggest.

Do the plants easily pull out of the ground?  If so, that could point to root issues.

Timothy mites do not impact orchardgrass.  And when we see timothy mites in timothy, it makes the timothy roll up, and gives the timothy a blue-ish green color - resembles a grass under drought stress.  The color you suggest does not mimic timothy mites.

Saw an insect problem in orchardgrass a few years back... I can't remember the name of the insect. However, I think it had reddish or orange body or legs (and I believe the opposite body part was black).  It caused the orchardgrass to die out - the appearance in the field was "patchy". This insect did not affect the field uniformly.

As I am sitting here typing, it occurs to me that if the problem is fairly uniform, then it most likely is not an insect problem. (would you agree?)

If you have some AMS handy, it might be good to spread some on a small patch to see what would happen - would some AMS help the orchardgrass pull out of its problem?