Rust color........I'd go up a couple of sizes for shits and giggles and see what the plugs do. Sounds to me, you might be lean..You want a chocalate brown color. The outside ring should be black and wet, the center strap should be dark brown and dry....There is only 1-2 sizes on the mains that'll yield this. Too rich and the strap will be black and wet. Too lean and you get different colors, grey, red, white, tan, etc...Lean conditions will hold back the revs, rich conditions will drop off fast and break up...