Ken, for daylight visibility blue is the way to go together with some blue or grey perspex as a contrast filter.
I have the same unit that you used, I ordered blue and they sent me orange, impossible to see going down the road. I guess red would be similar.
The blue one that I have now is easy to see and those units can be adjusted to account for the small voltage drop between the connectors in the headlight shell and the battery. Ideally you need a small relay and dedicated fused wire from the battery for monitoring.
They come in two variants , single wire that just measure what it's given as power and the two wire version that has the measuring wire seperately to power. Seperate measuring wire means v low current and no voltage drop on that wire. It's easy to convert from one to the other if you have a steady hand and soldering iron.
I had planned to wire mine up with a separate monitoring wire eventually, but at the moment it's doing its job. Just need to calibrate my brain to add 0.3v to the reading for now.
Should add that the contrast filter makes all the difference. Loads of suppliers on fleabay who will send you a 'sample' of 3mm perspex quite cheaply.
Regards
Dave
