I like the theme of color-changing, and am a big fan of what this card is supposed to be doing.

That said, if you cast this spell during (some) opponents' upkeep, this effectively says "Target player can't cast spells this turn." Alternatively, cast this during the attack step, then on your second main phase, your spells can't be countered.

Maybe that isn't a problem on just one card. But on a mechanic printed at common? You might be able to get one threat out and just keep raising the red tide on your opponent for five turns, dropping them to 5 life before they cast their first spell.

Thankfully I think you've misread this, jmg. It says "In addition to their other types".

I wish this said "colors and types" because it is really not intuitive that this overwrites colors, but just adds subtypes.

That said of all the cards that are flavored around blood in the water this custom keyword action certainly elevates this one above the rest on terms of flavor.

The set should come with rules defining all keyword actions and keyword abilities and stuff, so that would mention explicitly and precisely what it does.

It is true the reminder text is a bit unclear, but perhaps adding "(only)" after "red" would help if it is supposed to remove existing non-red colors. Explaining this in the rules with additional text also helps.