Green means you don't need heavy personal protection (like from a SCAPE suit) to work around the materials and there is much less potential for them to spread into the environment.

I handled hydrazine in a chemical fume hood. If that stuff spilled while a tank was being filled on a rocket it could potentially react with something nearby and probably cause an explosion or fire. Hydrazine is volatile like ammonia and would spread rapidly and get everywhere in the nearby area. You would need special gloves air tight clothing and a separate source of breathing air to clean up the mess.

The ionic liquid mixtures on the other hand are for the most part non-volatile and could be handled and cleaned up without a separate air source, although I would still wear protective clothing and gloves.

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Bruce S. Levison, NAR #69055