Color of spark plugs

We just did the annual on our 1945 Aeronca Champ.
Took the plugs out to do the compression test.
On inspecting them, noted that they were black, sootie.
The AI said that is normal for autogas in a Champ.

Honeck always says his plugs are clean when he runs autogas.
Is this the difference beween having a low compression, low horsepower,
no-mixture control engine and having a high compression, high
horsepower, mixture controlled engine?