They're both very similar, indeed. Under indoor lighting, they both look almost identical in colour, but in daylight, Newt looks visibly black-brown, and the tabby 'stripes' underneath her fur becomes quite visible. Newton is a couple of shades darker, but still doesn't look completely black (very close, though).
Newt also has a small spot of white fur just below her neck, and a larger patch of white on her belly between her rear legs (I think it looks adorable!). Newton, on the other hand, is 99.99% 'black' - certain areas of his body have random single strands of white fur sticking out, most of which are longer than the surrounding black fur, so they're easy to spot.
When Newt first arrived at my doorstep, she was quite skinny... a week before giving birth, she was still only 7.5 pounds. She has rounded out a little more and looks more plump now. Newton looks decidedly more lean and muscular.