Should someone say, "Whatever is a color is necessarily red,"... [we would respond as follows].
In this stylized indirect discourse, the grammatical subject of the verb say is the pronoun someone, marked here with the syntactic particle, rather than with the third case agent normally seen with this agentive-nominative verb.
The grammatical object--what is being said--is the pervasion statement of the incorrect position (Whatever is a color is necessarily red).
Although this passage appears to be an open-ended conditional construction (if...then...), here in this stylized rhetorical context it expresses a complete thought.