Spark plug reading

[1970gsx]

I believe a spark plug should have a golden brown or tan color if running properly, but mine are of a rust color. What does this indicate if anything? I've checked spark plug analysis webs but couldn't find anything on rust colored plugs. TIA

 

[Mr URL]

You haven't thrown in any octane boosters like 104+ have you?

 

[1970gsx]

No, I haven't used anything but 93 octane fuel.

 

[1970gsx]

??????