There are several genes that will give a snake a yellow color and enhance it. Usually pastel is the brightest, and that's enhanced with Orange Dream, Fire, or Yellowbelly. I'm thinking that could either be a straight Pastel or maybe a Pastel + Fire. The Fire gene will lighten the yellow and make it a bit brighter than just a straight Pastel. And there are many different shades of Pastel, apparently coming from different genetic lines. That's one of the brightest ones I've seen.