There is value to having things look like the colour they actually are outside of photography. Think of the nice red dress turning brown in the article.
Mixing colors in the array would be horrific. The array is sparse enough that each LED casts a distinct shadow through foliage. This forms a unique lighting texture on the pavement. If each LED were randomly red, amber, green, you'd have wild colors all over the pavement.
Single color is the way to go. And the broad spectrum produced by phosphor makes for better vision compared to the narrow spectrum of sodium or straight-LED lighting. IMO the best option would be phosphor + yellow or green LED.
Streetlights use arrays of LEDs anyway (to achieve appropriate brightness) so shouldn't have any problem using a mix of different colors.