Washington hauled down seven rebounds and Larry Nance Jr. added 14 points and seven boards for Wyoming, which improved to 8-0 for the first time since the 1987-88 season.
Josh Adams also reached double figures with 10 points for the Cowboys.
The Buffaloes (6-1) fell despite 24 points, six rebounds and four assists from Spencer Dinwiddie.
Andre Roberson added 16 points and 12 boards for Colorado.
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