"After I route my PCB, the D pads for the power transistors turn red.  What does this mean?"
Don't know.
Not enough information.

They could be single layer pads.
They could be something the Autorouter has done for whatever reason.

Unitl you can make a public copy of this project available and post the link to it back here, it's just guesswork.

Other points:

• It looks like you used the Autorouter on this because the routing is very poor:

given that you are using power transistors, the track widths to the "D" (and guessing your nomenclature, therefore to the "S") pins appear to be suicidally narrow.

• Your use of the phrase "D pads" implies that you are using MOSFETs Your PCB however slabels them as NPN and PNP which implies that they are bipolar transistors.

@andyfierman

Thank you for the response and the advice.  This is all new to me, but I am learning.

What I experienced was that each pad can be set to be to Top, Bottom, or Multi layer.
Both the G pad and the S pad on the MOSFETS were set to Multi Layer, while the D pad was set for the Top layer.  So it was Red.
Now I have them all set for Multi Layer, and they are all white.

And, yes, some traces were dangerously too close to pads and holes.  I  thank you for that tip.
I changed the clearance in the auto router design rules.  That fixed it.

Thank you for your help, it was enough to clear up the fog.