Typically in most other situations, the idea is to try to avoid making color the sole means of identification, but in this case given the sizes of lixes, you're probably not likely to be left with any good options outside of color. And then since there can be up to 9 colors that ideally should be all distinguishable from one another, I imagine it'll be quite difficult to find a good default that works well across the different kinds of colorblindness (to say nothing of not entirely sure on how to precisely define "sufficiently distinguishable"--I'd imagine with some hues becoming indistinguishable, you'll inevitably end up with some colors differing mainly by brightness/saturation and then distinguishability of those colors become solely a matter of contrast).
If we can be given mathematical formulas to model the various types of colorblindness as well as formulas for evaluating the distinguishability of two colors (and corresponding targets for pass/fail), it might be possible to entirely handle this problem via mathematical modeling and potentially come up with "optimal" default palettes that way.
Customizable colors is probably the safest approach and arguably a good requirement to consider--it ensures there is a workaround to whatever undiscovered deficiencies may exist for some people for whatever default palette we come up with. If you're worried about discoverability, you can potentially even consider making a one-time popup of a dialog for the first time the user goes to the multiplayer browser (and user hasn't changed the colors already previously), showing them the default colors and end with some text like "You can change these colors if you have some form of colorblindness and find some of the colors not distinguishable" plus a button to go to the relevant settings page (and another button for "this looks okay"). Or maybe instead just have an appropriately labeled and placed button in the lobby UI to go to the settings page for adjusting colors. You get the general idea.
Another interesting thing to consider is that the most important thing is for the color belonging to you to be distinguishable from the other colors, so that you can tell which lixes are the ones under your control. Likewise, it'll be important to be able to distinguish neutral lixes from your lixes or other lixes. The fact that a lix belongs to other player A versus other player B is often of less importance, except for team-versus-team style of playing. I'm not sure how all this translates in terms of how to approach the colorblindness problem, but I have to think it might help simplify the problem a little?