Bill,
just to say in DCC we use a Common Positive. So the blue wire is common to all lighting functions.
The white wire is a negative and goes to the front headlight Yellow to the other, green and purple (also negatives) for extra light functions
I have been putting one resistor on the Blue (common positive) because most of my installs only had front and rear headlights, when one went on the other went off. However if I used the two extra wires to run ditch lights or marker lights, I could have several lights switched on and sharing the one resistor. In this case I connect the blue directly to all led's and put a resistor on each light -ve.
Cheers
Rod

EDIT
Interestingly a US clinic advocating using a shared resistor, received mail advocating it was not a great policy, because if you have several led's hanging off a suitable resistor, they need to be a perfect match, otherwise one will take all the power and the other will remain off.
Although I should not have a problem with front and rear headlights where only one is lit at a time. The ditch lights would need to be another function though.