Mono gameplay:

Hit colors. Dodge greys.

Ninja Midway:

Never hit a grey to get a stealth bonus.
Grey blocks stay in the grid unless they reach the bottom, where they will be removed along matches or after a little while if no blocks are hit.
Power pellets remove greys from the grid, greys on the track are converted to color blocks for the duration of it's effect.
Overflowing takes away 50,000 points in addition to clearing the grid without scoring, try to avoid that.