509.1a. The defending player chooses which creatures they control, if any, will block. The chosen creatures must be untapped. For each of the chosen creatures, the defending player chooses one creature for it to block that's attacking that player or a planeswalker they control.
Quote from WizardMN »Multicolored spells are spells that are more than 1 color. Being *a* color is not enough; it would need to be something like Utter End for the Filter to apply.