What color should this be? It seems like it could be green, white, or red.

Most likely green, I think.

Mm, green certainly gets "Target creature must block me" I can see white getting something like "All creatures must block if able" but this is much more "You; throw yourself under the wheels of my monster" which green gets more.

Green is most likely to get "target creature blocks [something]]" (e.g. Tower Above), although red and white also got creatures with the keyword ability provoke. Just "target creature blocks [but it can choose what to block]"? It's a bit red (Invasion Plans, Grand Melee, Mark for Death), a bit green (Predatory Rampage, Provoke), and randomly bled into blue once (Courtly Provocateur).

Well, not quite random, random, thanks to Alluring Siren and Siren's Call... but yeah, I agree that forced blocking/attacking doesn't belong in blue.

To be honest, this creature is a bit of an enigma, eh? I don't think any of the colors are comfortable forcing a target creature to block like this, but Mark for Death is your best precedence. This would feel greener if it forced the creature to block this creature, sure. But 'target creature gets angry enough to block something, anything'? I'd say red.