So it's a 50:50 mix nitrite free, blue-green colour.
But, you could just top up with Distilled Water, technically it's reducing the ratio of antifreeze but not enough to worry. Distilled water is free of any impurities so nothing can go "off" within it.
The key bit of information I didn't find to post was this; they use a "ethylene glycol" long-term antifreeze. So it should be the same type as that. As opposed to propylene glycol.