I think the chances are extremely high that the cat did *not* come into its siamese (pointed) coloring from actual siamese parents -- he looks *nothing* like modern siamese (not even much the old-style applehead ones which a few folks are still breeding). Though if he turns out to have an unusually loud frequent *piercing* voice, sounding extremely different from most cats, then I may be wrong and he *could* be a Siamese x long- or medium-haired mutt cross, but I really don't expect that.
How old is he? If it is well grown then it is probably not a longhair as such (you know, the serious Persian-length mop type
) but rather a "medium hair" cat with the very dense plushy fine soft coat of longhairs without the full length. Sometimes the tail ends up plume-ier than the rest of the body, sometimes not. There are a few breeds for which this type of coat is the standard, but medium-haired cats from shelters are virtually always mixes.
I would guess that more likely this is a "mutt" with some Himalayan or Ragdoll somewhere back in its ancestry. Probably not as its parents, either, since it has neither white spotting (in photo anyhow) nor Himalayan-looking ears. There are lots of cats out there with 'siamese' coloring with no purebreds of any description in any recent part of their ancestry, so this is quite plausible for him. I would not predict any particular personality characteristics on the basis of how he looks, since IME cats looking like that can be 'all over the map'.
His color? It is hard to be sure from the photo but my best guess would be that he's a blue-point. Is the color on the bridge of the nose about what you'd expect for your basic solid gray (aka 'blue') cat?
I am sure he's a good cat -- if he *is* a longhair-shorthair mix, I have had great good luck with those, as they don't require as much daily coat care as longhairs but they're nice and fluffy, and often good lap cats
Good luck,
Pat