DC Wiring Color Conventions
First, I would tend to switch the high side for various reasons. 

The only convention I follow is to use "hot" colors for positive and "cold" colors for negative. And if there are only a positive rail and ground, use red and black for those.  

So with your wiring I might use red (+12)->orange->blue->black (GND)

With the contacts on the high side red->orange->yellow->black 

The color "hotness" changes where most of the voltage is dropped, across the loads. 



Maybe there is some standard that calls this out for DC circuits.