Okie dokie, I've been doing a bit of research, yes, even with my friggin' dial-up internet. I hope you appreciate my efforts on your behalf for this blog! (Do you feel appropriately guilty yet? hehehe...)
Okay, that's out of the way. That's my lame excuse for a joke, back to business...
I googled and did an ask.com search and it appears to me, that it's 6 of one and half dozen of the other about whether the color indicates temperament.
In my experience, I say, 'yes, it does' but a lot of the official research that I found refutes that. Usually I can find all sorts of experts in the horse field who know better than the rest of us but my personal findings differ greatly...and this is sure one that does!
It seems to me, overall and not in every case, that the wilder colored Appys with a base color of chestnut or red are a bit more opinionated than the subdued colored Appys. Of course, you have to take into account the bloodlines and how they've been raised/trained but overall, that's the way it seems to me.
Line-back duns seem a little more 'forceful' to me. Certain grullas are working sonuvaguns on ranches and very workmanlike and don't tolerate 'loving and petting'. Bays seem to go along to get along a lot of the time. Their skin seems to be thinner to me. Sorrels are tuff little rascals. I'm not sure about true blacks.
Then again, if you read the stuff on the 'net, the articles say it isn't so. One old-time article from over 60 years ago says that color does indicate temperament. I have friends who prefer certain colors for whatever reason.
Certainly, I can't solve the quandry here but this is something that interests me intensely. I'll keep checking into it and report back anything I learn.