The reddish areas, seen primarily in the spiral arms of m51, have that hue because:

The reddish areas have that hue because infrared light has been translated into colours we see with our eyes, such that the shorter wavelength is blue and the longest are red.

The reddish areas, seen primarily in the spiral arms of m 51, have that hue because clouds of gas are heating by ultraviolet (UV rays) light from nearby recently formed stars.