Well, Jolly Joker has been around for a fairly long time, it is entirely possible that there is a version with white in it as well. One of my favorites, Blackberry Sorbet (purple tops, black bottom) has a white variant, Blackberry Sorbet a la Mode.
It's a pity that you weren't at the nursery yesterday, they had LOTS of Jolly Jokers (in contrast I haven't seen Blackberry Sorbet for going on five years.)
Generally, however, the ones I like tend to not fit into any official named type. that's WHY I have such a tough time getting enough each year, I demand everything be more or less unique.
When I save seeds, I HOPE that they will prove true to type, and I will have the start of a variety of my own. So far, it hasn't worked out that well (either the ones I am interested in produce few or no seeds, or when they do, none of the seeds come up*) The closest I got was when I re-planted the seeds from the Blackberry Sorbet (which, since I had bought the better part of a flat of the plants I had a LOT of). And even THEN, only about 25% actually came back true to type (the rest either reverted to Johnny Jump up colors, or became solid purple)
Other could have beens included Cosmic Eclipse (nearly pure black, but with a sort of red-yellow x-ray effect around the center), Cat Whiskers (my general term for any pansy that is all white except for purple lines on the veins). Wine Dark Seas (a deep pinkish purple that showed up in a six pack of pale pink ones). Little star (the very FIRST pansy I ever grew successfully from seed, had a yellow flower with a black blotch, but both flower and plant shrunk down to the size of a wild field pansy).
My best hope for this year so far is a sort of purplish crimson.
* I have gotten LOTS of pansies to grow and make flowers (including a few that managed to overwinter their seeds and come up spontaneously in the garden. It's just that none of those were the ones I WANTED to re-seed and come back.)