The Out-of-Multiverse answer is partially in the introduction to the 1977 edition, page xi.:
In A Cure for Cancer, which is a kind of negative scherzo on the pattern of the first book, he has polarized into a black with white hair, and continues to vampirize those around him to maintain his own image stability. A couple of years have passed in the real world, the scene has darkened...
It's also partially is hinted at in several of MM's interviews. When Moorcock wrote the first draft of A Cure for Cancer it wasn't a Jerry Cornelius book. A friend pointed out that the main character was really JC, just in a different disguise. Moorcock, rewrote the book with Jerry as the main character, completing it in record time, apparently.
Symbolically, Negative-Jerry is the emotional and psychological opposite of Normal-Jerry. He says it himself in the first couple of pages of the book.
The In-Multiverse answer is a little more complicated and relates to the concepts of the "Eternal Champion" and the Guild of Temporal Adventurers. The appearance of a multiversal traveler depends very much on the universe and the role in that universe. So changes in appearance, even dramatic ones, are to be expected as one travels through the mega-flow.
(1977, page 175) Jerry dresses and views himself in the mirror.
A very negative appearance, he thought, pursing his lips and smiling.
Later in A Cure for Cancer, when he is recognized by Bishop Beesley and Frank (his brother), they comment on his change of appearance and attitude, however they don't seem to think it particularly unusual (especially for Jerry).
(1977, page 201)
It was a shame they hadn't put the light out. Bishop Beesley, peering through his spy-hole in the room above, frowned. He had recognized Cornelius.
(1977, page 215) Beesley seems more surprised that it isn't a disguise.
(Beesley interrogating Jerry for the first time) "White pubic hair. I hadn't expected that...
(1977, page 332) Frank doesn't seem to notice the change at all
(Frank C meeting Jerry in the US) "Long time no see, old chap!"
(1977, page 416) Jerry even changes back at the end, in a particularly symbolic moment.
(Catherine, just after Jerry resurects her) "You've turned quite pale." Jerry: "It's for the best I suppose."
As an example the eternal champion having a different appearance, Erekose, has totally black skin.
http://en.wikipedia.org/wiki/Erekos%C3%AB
In another example, in Moorcock's book Blood, Jerry is a minor character described as an African-american.
https://www.kirkusreviews.com/book-reviews/michael-moorcock/blood-2/
The obvious opposite is the albino Elric of Melnibone, who is pale, physically weak, self-pitying and depressed, much like Jerry at his worst.
