I get that the OP has this tip that helps him out, but this is a crummy plaster solution to a much bigger problem on so many printers, with operaters not caring to get educated on how to fix the problem. Again, not dissing the OP, but this is not moving a the problem towards a good, sustainable solution.
What if the file is being viewed close up? What if the image contains grey areas within a larger colour image? What if it’s an actual logo? What if you’ve been supplied a massive flattened image?
Rather put time and money into sorting out your profiles. If he is running a 315 then the profile is very possibly not correct for the media (remember that white comes in many colours) so you might be thinking a profile for a white cast is fine for a cheap promo white vinyl. But it’s not. Cheap vinyls have been whitened to within an inch of their usability with OB’s. HP actually ships with really good profiles, so I doubt it’s their profile that has the issue.
Check if the light colors are holding up during printing. The light colours on latex machine have been historically shitty and that side of the head would fail way before the others.
The biggest issue I have with this tip is that if your greys are out, then your whole image is very possibly out. Greys are the first thing I look at after doing a profile. How is this solution fixing all your other colours?
I’m willing to help you create a proper profile, just keep in mind that I am based in South Africa, so sending printed swatches will take a while.