there's has yet to be a person to claim that they can tell the difference between a single YB, Gravel and Asphalt. even the big, famous breeders can't.
Actually, this is probably the key difference between them - people can and have been able to distinguish Asphalt from YB while Gravel and YB consistently have everyone stumped.
Aside from that, as has been mentioned, the superforms are somewhat different (SuperAsphalt is more banded whereas SuperGravel is more striped), and the Freeway and Highway are visually different (Freeway tends to retain some darkness to it as it ages whereas Highway tends to pale out more)
I can not see the difference in asphalt vs gravel vs yb with any certainty but highways and freeways are easy to tell apart imo. The pattern is very similar but the color/contrast is way different even from a young age. Everyone I've seen in person I could tell right away if it was highway or freeway. It's hard to describe the difference but the most obvious is the contrast and difference in the darker coloring. In freeways, the darks are darker making a much higher contrasting snake. The highway looks like a faded version. Asphalt and gravel are not like the butter/lesser morph which is the same, gravel and asphalt are different morphs based on how different what they create is.
Gravel is rock and dirt. Asphalt has tar mixed in it....
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