My fountain pen recently ran out of ink and i decided after cleaning it that now is a great time to mix some inks and try to find a new color i like. So i picked up a bottle of red (Corail des Tropiques) to go with the green (Vert Olive) and blue (Bleu des Profondeus) i already had. The photo above shows the colors of these inks, alone and mixed.
In case my handwriting isn't clear enough, on the right of the page (top to bottom) are Vert Olive, Bleu des Profondeurs, and Corail des Tropiques. All from Jacques Herbin, since i thought it would be safest to not mix different brands. On the left are green & blue, blue & red, red & green, and then all three mixed.
The green & blue mix has a bit more green than blue, and the blue & red mix has more blue than red. Still, these turned out to be harder than i expected to measure precisely. I guess that comes down to me using a pipette instead of an eyedropper. I just mixed the inks in three small jars, two colors in each jar, and then painted them onto a piece of cardstock with a small brush. The last one was done by mixing the three test jars after i had samples of each.
My favorite of these is the purple i got from blue and red, followed closely by the dark gray from mixing all three and then the darker green from green and blue. Blue alone is also fine, but i don't care for straight green or red, or those two mixed.
Purple was the least surprising result. I figured green and blue would make something more like teal. Maybe they would have had i used more blue or started with a darker green. The brown i got was more orange than i expected, again because the olive green is so close to yellow. What surprised me most is what i got mixing all three. I assumed i would get a muddy brown but it came out as a lovely light black^1^ instead. Reminds me of storm clouds in low light.
I did ask at my local nice stationary shop about mixing inks, but nobody working then had done it before so all they could say is that it would probably be fine. They did wonder if the inks would mix completely or if i'd end up with a multi-color ink that changes as i write. That didn't happen, they mixed very easily. I could probably get a multi-color ink if i just added different colors to my pen without first mixing them, but they'd still mix together over time and as the pen moves. I imagine marbled ink would work better for a dip pen that can write without stirring up the ink much.
Filling the pen
Filling the pen with my new purple ink was harder than i expected. The jar i mixed it in was too wide (and thus too shallow) to fill it normally. I have a TWSBI Eco so the normal way is to stick the head of the pen into a jar of ink and draw it up with a plunger on the back of the pen. Instead i took the plunger mechanism out of the pen and filled it there with a pipette. I had the pen sitting nib-down in a jar of water in the hopes that that would help stop ink from leaking out before i got the plunger back on. It did not, and also some water got in and i had to scribble for a minute to clear the watery ink and get to the stuff that's visible. What i should have done is set the pen in the jar of ink i was filling it from, so any leaked ink would not be wasted. But now i know for next time.
Notes
^1^ I distinguish dark and light gray from light black and dark white. Dark white is the color of snow at night, light black is the color of an overcast night sky lit up by light polution. Light gray is the color of dusty gravel or clay, dark gray is the color of Batman.
Test tubes are really worth it is you want to mix inks often AND want to use them to fill a piston pen (or vac, sac, etc), but if your jars work, they work, plus they can stand up straight on their own.
Well, Herbin is, as far as I am aware, the company that has been making ink the longest (228 years so far), so they have quite a bit of experience. Prior to that they were already trading in pigments and making colored seal waxes, so even then, they didn't start from scratch. Depending where you shop, they can also be super cheap, if you buy the plastic 100mL bottles of their normal inks (not shimmers or special editions), they can be had at around 13-14€ depending on the color, so still cheap AND very cheap per unit of volume. Though not all their inks exist in that format (Bleu calanques for instance isn't made in 100mL).
As long as you mix inks, and not dyes, and dye based inks only, even making 1L at a time shouldn't be a big deal. I mean... it might get cumbersome or tiresome if you go overboard eventually. For separation, it can happen that you'd get some color shift once inside the pen, after resting for a while (like a day or longer) one of the inks you used shows up first when you start writing, then it turns into the proper mixed color, but there is no way to tell whether it will happen until you try, though keeping to a single brand usually prevents this. And even shaking the pen wouldn't prevent that because it happens in the feed for the most part.
But if we're talking about mixing ink from dyes (or even worse, pigments), then this is when the stirrer comes in handy, because dealing with mixing pastes to water is rather time consuming, and doing it by hand wouldn't yield good results, because not only do you risk having clumps of dye floating in there, but you wouldn't be able to have a predictable hue if you haven't mixed in all the dye.
I have had somewhat bad separation when trying to make a pigment based warm, dark brown. I used a red, a brown and a black, and it acted really odd: the center of a stroke had a streak of pure black, the edges had the red come out and sometimes even feather out alone, then the rest was proper brown. Shaking the pen before writing limited the effects, but depending on the paper, the black streak or the red feathering would still always come out. That batch made me give up mixing pigment myself, it's too annoying to deal with as a DYIer.
Human eye color sensitivity is only relevant to plan out your mixes, or understand what will or has happen, nothing further.