The solid color to dotted design for the red line was originally done for television. On early black and white TV broadcasts, it was hard to differentiate the blue lines from red lines (they all just looked dark grey) so the League mandated that the center red line should be dotted/designed. It's actually an official rule (NHL Rulebook 1.5: "...This line shall contain regular interval markings of a uniform distinctive design, which will readily distinguish it from the two blue lines, the outer edges of which must be continuous. (Paintcode PMS 186)".