With the Rockies trailing 4-3 in the eighth, Carlos Gonzalez drew a two-out walk from Sam LeCure (1-1) and Tulowitzki followed by smashing a 3-2 pitch toward the left-field wall.
A fan wearing a glove reached out over a ledge to grab the ball on the fly, prompting a video review. The umpiring crew, however, upheld the call and Tulowitzki was awarded his 13th homer of the season.
Jonathan Herrera went 3-for-4 with a two-run double as the Rockies evened their three-game road series.
Josh Outman (2-0) earned the decision after getting the final two outs of the seventh, Matt Belisle tossed a 1-2-3 eighth and Rex Brothers notched his second save for Colorado.
Ryan Hanigan drove in two runs and Todd Frazier went 2-for-4 with an RBI double for the Reds.