Through September, U.S. sales of the Silverado jumped 15 percent, making the Silverado the only one of the top five full-size pickups to post double-digit sales growth (followed by the GMC Sierra, up 9.8 percent; Ram, up 3.4 percent; Ford F series, up 1.3 percent; and Toyota Tundra, up 0.9 percent).

Just 12 percent of Chevy Colorado buyers traded in a Silverado in the 90-day period from May through July, IHS Automotive data show. The rate of Sierra-to-Canyon converts was even smaller, at 9 percent.

"Everyone thought that the Colorado might eat into Silverado sales," IHS analyst Tom Libby says. "That hasn't happened."

The top market for the Chevy Colorado is California. Its success there "all of a sudden starts to create some momentum around the brand" in a market where Chevy long has struggled for a foothold, Chevy global brand chief Alan Batey said in an interview last month. In the Los Angeles metro market, Chevy's overall sales were up 16 percent through August vs. 10 percent for that market as a whole.

Chevy's marketing play for the Colorado took the brand out of its usual comfort zone -- think big sporting events and Texas -- to target active, outdoorsy buyers who might identify more with Subaru than Silverado. Over Labor Day weekend, Chevy sponsored the U.S. Sand Sculpting challenge in San Diego, for example.

Competitors are responding. Toyota is taking dead aim at the Colorado's active-lifestyle psychographic with a new advertising campaign that features young people tearing through the desert under the banner "Play Now."

And old Detroit 3 rivals are threatening to jump back into the midsize pickup market, which could water down profits in a segment that's still only one-sixth the size of the full-size pickup market. Ford is considering a revival of its Ranger for the U.S., while Jeep plans a Wrangler-based pickup.

Those trucks would be at least a few years off, though, leaving GM execs feeling pretty good about their decision.

"The pundits and manufacturers questioned the soundness of our plan," says Sandor Piszar, Chevy's director of truck marketing. "Now, we see folks scrambling to catch up."