I've been looking at the fan art for Good Omens online, because when one searches for comments, they are usually mixed in with fanart. Much of it isn't to my taste but some I like, and some of the spinoff cartoons are funny. (Search for Crow-Ley and Aziradove, and also Abziraphale, which are generally safe, but remember that rather a lot of fan art is NSFW and it isn't always marked or behind a cut.)
There is a bit of a disagreement among fan artists, though most are quietly happy to draw their own versions and let others do the same. Disagreement happens because the book gives us hardly any description of the two main characters. Crowley has dark hair and good cheekbones, looks young (or at least younger than Hastur), is lithe, and wears dark glasses. Aziraphale has plump elegantly-manicured hands. That's all we get, aside from a few comments about 1950s style clothing.
There are a few authorial head-canons: Sir Terry suggested that Aziraphale might be played by a burly blond actor, who was muscular and bulky, but not obese. Neil Gaiman suggested a younger, thinner actor who was also blond, and specified "blonder". So they both thought of Aziraphale as blond, but different body types. (Even those head-canons are not set in stone because they both thought the late Peter Sellers would have played either role well and he had dark hair.)
Almost no human character has even that much description, except Pepper, who is a somewhat cliched fiery redhead. We have more description of the Four Horsepersons.
Fan artists have drawn Novel-versions (black hair on Crowley, tartan and sweaters on Aziraphale), Radio-versions (similar but generally thinner), and Series-versions (red hair on Crowley, plump Aziraphale), with costumes according to the cover pictures and radio descriptions, but many have gone on to develop their own concepts. Many choose to draw them as POC, and while that startled me, it is true that there is nothing in the book to preclude that imagery. History might indicate difficulties had they been dark skinned, but there's no solid reason against it. TV being a visual medium, actors had to be cast, and the Series cast is diverse, with Pepper played by a fiery young black girl.
I do find it harder to like pictures of skinny Aziraphales, because that is the one bit of description we have, that he is plump (and elegant). Not massively obese, just comfortably padded. There is even historical support for that. Aziraphale spends far more time involved with humans than Crowley does, especially after Heaven begins monitoring his use of miracles. He seems to spend most of his time in Western Europe and the UK, locations where life was generally much easier if you were not only male but white, and blond hair was a definite plus socially for several centuries. Being obviously well-fed was another indication of wealth and therefore power. It makes sense for Aziraphale to be plump. (Also, angels can shape-change. He could be skinny if he wanted to.)
Looking middle-aged is another useful camouflage trait for Aziraphale. In most centuries, young men would be expected to be involved in military activities, or courting and raising families. By the mid-20th century, wearing glasses and looking older and visibly out of shape would have helped keep him out of active service without being hassled in public, though he would have been expected to participate in home defense activities.
On the other hand, Crowley doesn't need to convince anyone of anything. He can use whatever shape he prefers, because he doesn't associate with any group of people very long, just long enough to do a temptation and maybe have a few drinks. Nobody limits his use of magic, though they apparently check for types of miracles once in a while, so he can create any costume he wants in order to blend in just long enough. Nobody draws him plump, though...