Foxes have a solid body colour with pea spots behind the ears and some ticking on the body. The tummy is like the Agouti's- a different colour to the top colour eg Silver. They also have eye circles in the same colour as the belly and theit chest is the same colour as the belly too. They were introduced about 6 years ago with the Black Tans.
Silver Foxes are lovely! They are a fairly new breed and I believe came originally from Switzerland, along with a similar breed, the black tan (which is black on top with tan underside). The foxes and tans also come in other colours (eg lilac fox / lilac tan; choc fox / choc tan). All variations are really beautiful.
You can also get a 'black otter' guinea pig, which is similar to the fox but the markings are cream instead of white (like the black otter rabbit. Here's some pics of a black otter boy I used to have, as you can see he's also crested:
I have a chocolate otter boar. His markings are leaning more towards buff. I prefer buff markings on them as cream is too light and too close to white as in the fox. Buff is a happy medium between white and tan