That would be an RO unit.. RO/DI has a final DeIonizer stage added on. If it is used you can buy everything you need to refit it reletively cheap. (hope you did not pay an arm and leg, they can be pricey) There are a number of site sponsors here that sell filter packages for RO units and there is always ebay if you feel lucky. I would suggest a 10 micron carbon block, then a 1-5 micron carbon block, then a 75 GPD membrane unless you need a massive amount of water. You can buy a new flow restrictor with your new filters. I am sure one of the sponsors on here that deal with RO unit would be more than happy to go over all the fine details...
From the picture it looks like red is feed, green is cleanest, and black is waste (which is still fine for laundry, drinking, and plants). When you hook it up the most water pouring out will be the waste water line. Remember when you start a run to dump the first few minutes of water every time, that will be dirty or stale from not running... that is especially important if you have a DI stage and don't want to be changing your DI resin a lot.