Keep in mind that a different relay will be needed to run the turn lamps as they will flash in a manor most likely to cause convulsions in the traffic behind you.
It's more efficient for the LED to match color of lens.
A red LED puts all it's output energy into the red portion of spectrum ... red lens allows red to pass thru and blocks other colors (of which there aren't any in a red LED).
A white LED has only a fraction of its energy in the red , the rest is green & blue (red+green+blue light = white to the eye)... thus a red lens blocks some of the white LED's energy output.
All else being equal, a red LED should look brighter than a white LED, when both are viewed thru a red lens.