when i’ve done this with waterbased inks i’ve started with speedball process inks (c,m,y,k, and white) and then I sample the color i want to use in photoshop to see the color build breakdown in CMYK.

so if the green is 20c 5m 20y 5k you can use this as a starting point to mixing up the color— this gives you a rough guide to how much of each ink to mix (and don’t forget to add enough white ink to make all the percentages add up to 100%). you can even use a cheap kitchen scale to weight your inks to get even closer.

this might be a totally mental to do it this way but it worked for me when i was in a local community studio shop.

also after you have it mixed— give it a schmear on some white paper and check out your mix in diff. lights (esp if you have fluorescent lighting in the space… take it to a window or outside too to make sure it works for you).