Why Do D.C. Metro Police Wear Blue Helmets Like Those of UN Troops?

Here are pics of Washington, DC metro police with sky blue helmets on their heads:

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Sky blue is the color that United Nation troops wear on their heads, whether helmet or beret:

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To my knowledge, DC metro police are the only police in the U.S. who wear the same blue-colored helmets as the UN.

Here are pics of the police riot gear of three U.S. cities: Boston, Dallas, and Seattle. Notice that their helmets are all black in color, not blue.

H/t FOTM’s Miss May

~Eowyn