This was the most fun I've had actually playing something in this jam.
The look is fantastic. Title screen with the logo in purple and pink graffiti over a paint splatter, ninja throwing the V sign, and then the game itself drops to pure black and white with only the yellow of the cans and the blue paint you leave behind. Very stylish and very readable at the same time. The combo system is addictive, watching TAG + TAG + TAG + TAG stack up at the bottom with the multiplier climbing (900x1.2) got me to 1500 points quickly, and COMBO FAIL! in red graffiti is clear feedback with personality. The little drawn keyboard on the title screen with GRIND (R) and JUMP (SPACE) marked does the job, and crediting the music (Street Eaters, Witch) discreetly in the corner is a nice touch.
Three things. The black and white contrast is beautiful but the character sometimes vanishes into the black background, especially when you drop to the lower part of the screen. I never quite worked out when the grind actually engages, sometimes I would pass right over a rail and not catch it. And the level is massive like you say, but with no minimap or direction hint you basically run right and hope.
A 20 second tutorial teaching the grind and the wall jump would make this an easy favourite of the jam. Really good work.