I'm a geneticist so hopefully I can explain this is a way that everyone understands, rather than just what makes sense in my head.
Fun semantic fact for you: when people say "roan gene" they actually mean "roan allele" much like when people refer to the "breast cancer gene" they mean "breast cancer allele". All guinea pigs have the gene responsible for roaning, it's the same gene responsible for non-roaning in fact, but not all guinea pigs have the variation (allele) of that gene that produces roaning. Phew.
But yes, Soecara is right. The roan allele is co-dominant, which kind of means it vies for expression with the allele for non-roaning, resulting in variable expression. Which means a pig can have patches of solid colour and patches of roaning, which as you can see in the pictures of those very cute little bubs is a grizzled appearance, made from hairs lacking pigment and therefore appearing white interspersed with other coloured hairs. You do run into issues with pigs with normal white patches, where roaning can be, I guess I'd say, cryptic. Even if you know the parents of a white piggy, crossing a roan pig with a non-roan pig results in a 1 in 2 chance of babies being roan. So the only way to know if a piggy with white on it, where one of their parents is roan, and there is no visible roaning on the white piggy, is to breed that white piggy with a piggy that definitely isn't roan and see if any of the babies end up roan. Obviously, genetics is a roll of the dice so there are no guarantees you'd get a roan piggy even if one of the parents is roan, so really you might need a couple of litters before you can be more sure. I'm exhausted just typing that, so I can imagine what the pigs must feel like after all those babies.
Lethal piggies result when both parents are roan, having one copy of the roan allele each, and then they both pass on that roan allele, resulting in a baby with two copies of the roan allele. One of the fascinating things I find with lethal white piggies is that they are white because of the double roan allele (called being homozygous for the roan allele), which affects melanocyte expression a great deal. Which means they're only white in appearance, lethal white piggies can be genetically black or chocolate or gold or agouti or any other coat colour that exists in guinea pigs, it's just that double hit of the roan allele prevents that colour from being expressed. I always find that really interesting to think about anyway.
Sorry for the small essay, I just woke up and am feeling uncharacteristically enthusiastic.