First off, it sounds to me like if you're serious about this you really do need to learn at least the basics of color management.
As far as your problem, it's a profile issue. Just as simple as that.
The first and most important lesson to learn about large format inkjet printing is that Profiles Are Everything.
They're everything because what a RIP does -- its actual, important function, not imposition or other add-on stuff -- is to convert pixel information into dot information using information in a profile. In other words, the profile tells the printer what dots to print.
And black turning to green is actually a pretty common problem. It's particularly common among vehicle wrappers who'll print a black that looks pretty good inside, and then they get it outside and it turns a weak greenish color. The issue can always be solved with a proper profile. But to me, that doesn't mean anything you can download. It means a profile made in your environment on your machine on your media and expressly for your needs.
That's number one.
Then I'd say you need to understand what a color managed workflow is and how to achieve it.
And that means understanding that you can't just say "RGB" and "CMYK." You have to know which RGB and which CMYK.
What I'd say just to kind of point you in the right direction is that typically, RGB color spaces are larger than CMYK color spaces. And it's almost certain that the color space of your Epson has a much larger gamut than the default CMYK color space in whatever applications you're using. So your workflow should always be to work and edit in some RGB space, to tag your work, to set up the RIP to expect your work in your working space, and then to let the RIP convert to your printer profile at print time.
Bottom line is that these days, the process of printing digital color has gotten so advanced that if all you're after is a relatively pretty picture, just about anyone can get there right out of the box and with very little effort.
But if you're serious about doing it right, the learning curve is still steep, and there's only one right way to do it.
Mike Adams
Correct Color