I would suspect that they yellow ink has gone off a bit, or at least that your printer + profile + driver isn't handling the yellow values well. It can be very difficult to tell without a print colorimeter, since we humans are not very well equipped to see "dark yellow"—all saturated yellows look pretty bright to us.

The standard quick tests aren't going to work well with your printer since it uses dedicated red and green inks (colours that would normally be mixed from magenta + yellow or cyan + yellow in a CMYK print). You can try printing a solid, vivid orange—that will have to be mixed from some combination of red, yellow and possibly magenta, so if that comes out too red, then the yellow density is the problem. That still won't tell you whether it's that the yellow ink itself isn't strong enough or that the printer is applying the yellow too lightly, though. Flushing and recharging an ink can be an expensive proposition with a CISS due to the line length, so it's not something you'd want to do on a whim. Try the orange test first.

It may be less expensive to have a custom profile built for your printer than to replace an existing ink stock on speculation. Cathy's Profiles is a good source (good enough to get a recommendation from Ctein†), and at $35 (current price) it may save you quite a bit of money compared to replacing an ink. Having your printer re-profiled whenever you change inks, especially if you are using third-party inks or papers, is always a good idea.

† Although Ctein doesn't appear as an entry in Wikipedia, he's all over the citations for various photo processing and printing topics. He's also the author of Digital Restoration from Start to Finish, which is where he's recommended Cathy's Profiles for those unable to own something like the ColorMunki Photo.