Protection from multicolored means the protected creature cannot be damaged, enchanted/equipped/fortified, blocked, or targeted by anything that is multicolored (the DEBT acronym). A card is multicolored if it has two or more colors. CR 105.2 defines multicolored as having more than one color, and CR 702.16b defines what protection prevents.
A spell or permanent that is both red and blue has exactly two colors, making it multicolored by definition. Protection from multicolored therefore applies to it in every relevant way — the protected creature cannot be targeted by such a spell, blocked by such a creature, or damaged/enchanted by such a permanent.
It does not matter which specific colors the multicolored card contains. Red/blue, white/black, five-color — all of these are multicolored, and all are equally stopped by protection from multicolored.
Example: Your opponent controls a 3/3 creature with protection from multicolored. You cast Electrolyze (a red and blue instant) targeting that creature. Because Electrolyze is multicolored, the creature has protection from it — the spell cannot legally target the creature. If you somehow forced the targeting, the spell would be countered on resolution due to an illegal target.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.