what is the significance of the colour RED in the poem "Telephone Conversation"?

The poem "Telephone Conversation" is about the politics of colour, which here signifies race. The conversation is between a man of African origin (the speaker) and a white woman whose room he hopes to rent. When he confesses that he is black, the woman wants to know what his exact colour is ("Are you dark? Or very light?...like plain or milk chocolate"). The poet, to emphasise upon the outrageousness and extreme prejudice of this attitude, brings in the colour 'red' in its utter banality (where it occurs in day-to-day London reality on omnibuses and phone booths) but also in its profundity (the colour of blood in people of all races is the same).