Air needle

Mine are the same color.

 

The primary will air up first then secondary

 

I say green. My Pete has a separate guage for each.

 

It depends who makes it. It's not a decision maker nor a rule. Read the owners manual.

The primary will fill up first and the needle will sit higher. Green usually means everything is okay while red is a warning. That said red is your last ditch effort to get pulled over. That's my guess.

FMVSS No. 121

 

Don't worry about colors too much.. but DO know what they "usually" read. Some truck/trailers, depending upon HOW they are plumbed...and what valves may..or may NOT have been replaced correctly ( just because it looks the same doesn't mean it has the correct 'cracking' pressure ). Big differences exist. You should find that when the truck is completely OUT of air...the primary may fill first, with secondary following & slower. Don't touch anything until the governor cuts out the compressor. You now know the tanks are full. Wait...the second you release(charge) the tractor parking brakes, the pressure will go down. Increase RPM's until the governor operates again & you hear the cut-out whoosh. Now...the tractor ( assuming it is) has full tanks & your parking brakes are released. Charge the trailer...same thing will happen...pressure will go down. Here...depending on what kind of trailer, & the plumbing you've got...& the trailer system integrity...it may take no time at all ( if it holds air )....or if you happen to be running a belly dump or anything with large air-operated items it may take minutes to fill the system enough to 'allow' the parking/spring brakes to release. Think about it. Each vehicle has multiple tanks, and it's likely they fill individually with protection valves in each tank/circuit. My experience is generally no problems w/a van or flatbed. If you get to know how the system generally fill,etc...I think a person might be quicker to notice a potential problem.

Maybe too much info....but I've seen a co-worker leave his truck on the jobsite trying incessantly to charge the trailer every time the tractor built over 65-70 PSI. Left it there, convinced he had a bad compressor.
I tried to..ah well nah...he knows better then me. Took my unit home...while he waited for a soon to be unhappy road call mechanic.

 

You've got all I can comment on & know I'm correct. As I said, relay, check, & other valves have different operating pressures, & folks change how the systems are plumbed. Sounds like maybe a check valve isn't allowing one system to charge the other. Sometimes they have it set so if you lose air, the secondary will give you enough to get off the road before the maxi's ( spring ) brakes set up. With air you should be able to move. Both tractor & trailer locked...? Find someone who KNOWS the systems. There are charts....but I have seen so many minor variations it's not funny. Is your MV-2/3/4/ dash valve leaking at all? I've seen air back feeding to the spring brakes 'n other problems that made diagnosis difficult too. You have a Harvester? What exactly does the truck haul/pull? Just a flat or tank/van? More info plz.