Is the engine done for?

Honestly it depends, how bad is the oil filter housing leaking? And how often do you have to add oil to compensate for said leak?

Greyish colored motor oil often indicates water/coolant mixed into the crankcase. How are your other fluid levels? Did you check the oil level or is your ride one without a dipstick? If so, you might be able to purchase one inexpensive to check your oil. I would presume that if coolant is mixing into the crankcase, it would be reading overfull. The Oil Pressure light could be cause of a faulty pressure sensor or due to low oil pressure because of worn bearings the only confirmation truly is a mechanical oil gauge compared to specs.

Yep, "it sounds like you have a blown head gaskets leaking coolant into the crankcase where its mixing with the oil. hence the milky oil. If the car wasn't driven too far since the leak began then the engines probably okay, but it can be a costly repair. here is a tip "if the engine still idles well enough. the compression is still strong enough even with the leaky gasket. so, it might be worth repairing. hope this helps good luck.

Best thing now would be a leak down test or even a simple compression test to at least try and isolate the area where its leaking from, but keep in mind sometimes it turns out to be more severe like a cracked head or block but most times it's the head gasket. Good luck.