Do you mean EC film? Those are only available in the same colors that the sheeting itself is available in so if he's trying to hit a different color(that's not available in film or sheeting) then this wouldn't work.
I was thinking about the spec issue. You're not going to meet specs with a wide format solvent print/ink unless your state is drastically different than mine.
Thats the issue I see, all states have to comply with MUTCD if they are funded by the fed. Some states have a stricter traffic code, even city have their own. Some smaller city wayfinding
sign projects use various colors outside the translucent film choices 3M has, that is usually screen printed. every time I have seen a digital print, they failed or faded.
If you are not up on MUTCD than your bigger problem might be knowing it, than trying to find a way to safely print on digital media. There is a project out there that failed miserably because they tried a an unproven print process. If you are using 3M filMs, contact 3M, then get the specs in writing, then submit that to your client and cover your butt... IF this an actual traffic/MUTCD job.
First I have seen of the Durst machine, going to have to look into some samples.