Fading red to white to blue continuous loop on ws2812 LED strip
Think of a color as a point in 3d as defined by its r g b attributes 

Think of two points here, one is your starting color in that 3d space, and another your ending color. 

Figure out the path you want to take from the starting color to the ending cokor.

Chop it up in a few steps and use your code to walk the path.

Done