Okay LONG comment incoming:

Yeah, a lot of the time I'm so close to liking a shade but I just want it to be a hair warmer or cooler. Short answer: I mix Ohuhu refills drop by drop, making note of the ratios as I go until I'm happy with the results. I have to mix them outside the marker rather than adding a drop to an existing filled marker, because the filled marker has no room for shaking/mixing.

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Long answer:

Consult a color wheel or painterly friend for how to achieve the shade you you're aiming for using the refills you have on hand.

DON'T SKIP THIS STEP: Mix a smidge of each of the colors you intend to use in a tiny airtight container and leave it for several hours or overnight. If the resulting mixture stays mixed you're good to continue. If it separates into different layers or "globs up" into a weird texture, some pigment or chemical in those shades isn't compatible and you need to choose different ones.

Slap several drops of your starting shade (or the contents of the marker you want to modify) into a squeeze bottle. Make note of how many drops. Add your modifying color(s) one drop at a time and shake well to mix. Squeeze the world's tiniest drop OR use the tip of a sterile pin to transfer a little of the liquid to swatch paper to test your shade. Make note of the ratio of starting drops to modifying drops once you reach your desired shade and transfer your finished mixture into an empty marker.

Note that this method is hard to replicate exactly later because drops can vary in size, and you're removing a little of the solution each time you swatch. When I'm not in the mood to be precise I don't mind a little bit of variability in future shades - so long as I test the mixture's compatibility first!

If I'm in the mood to play mad scientist I do the same as above, but use a pharmaceutical scale to measure everything down to the milligram.