While Himalayan suppresses the phaeomelanin on the entire body and the eumelanin on all except the coldest extremities, the Californian doesn’t suppress any phaeomelanin, and only suppresses the eumelanin on all except the coldest extremities. That might be a little more in-depth than you wanted to go, but that is the best way to explain it
Californian markings can’t be on any self color, they can only be on a red based color. They have dark eyes IF the smut (nose, feet, and ears) are black or chocolate. If the smut is beige or lilac, the eyes will be red. There would be no real difference between a white beige or lilac Californian and a beige or lilac Himalayan. Himalayans always have white bodies and they can have black, chocolate, beige, or lilac points.
Red x Himalayan would give you, more than likely, black. Red AND Himalayan on the same animal are what causes our PEW, which is not actually albino like they are in many animals. Himalayan removes all red (phaeomelanin) while red or non-extension removes all black (eumelanin) – combined that removes all color from the coat, leaving white.
Another difference is that Himalayan is recessive (ch) – the most recessive allele in the C series. Californian, on the other hand, is dominant (K). You must have a Californian parent in order to get a Californian offspring, but two non-Himalayan parents could produce a Himalayan baby (often blacks are used, so two black parents could have a litter of Himalayans).