Why can I see the infill through my translucent ocean colors on this color-mix map?

Your instinct about the pattern is right. Grid infill is the worst choice for see-thru filament because every one of those straight lines runs wall to wall in a straight shot, so light travels straight through them and you get those vein-looking streaks. I ran into the same thing on a translucent green lamp shade I printed last year. Switching to gyroid made a huge difference since the wavy pattern scatters the light instead of giving you long continuous lines to look through. It's a bit slower to print and uses slightly more material at the same percentage, but for a display piece like a map that's an easy trade. If gyroid alone doesn't fully hide it, bump your top solid layers from the default up to 5 or 6. More solid layers on top does more than extra infill percentage does, honestly, because the infill is still visible through a thin top skin no matter what pattern it is. You could also try bumping the layer height on the top layers only if your slicer allows it, thicker layers hide the infill better.

One more thing that helped me with translucent colors: dry the filament. Translucent spools that have sat out a while print slightly hazy, and moisture makes the see-through problem worse because you get tiny bubbles scattering light everywhere. A few hours in a dryer or the oven at a low temp cleared up my blue noticeably.

On the purge bin question, yes, I let it ride past the warning all the time. The prompt comes way earlier than the bin is actually full, it's basially a conservative nudge. I've done long multi-color jobs where it asked twice and I kept going both times with no spillage or issues. Just peek in there when you walk past the printer and use your judgment. If it looks more than about three quarters full or the poop pile is getting near the top edge, then pause and clear it, but otherwise there's no need to interrupt a two-day job over a nag message.

Gotta disagree with the advice to just ride out the purge warnings. I ignored that nag once on a long job and the pile kept stacking until the later poops started landing on top of the old ones and sliding off toward the bed. Lost a night of printing to that cleanup. The warning might be conservative but the pile grows fast on color changes, so I'd at least check it when the prompt shows instead of trusting it'll be fine.

Another thing that worked for me on a translucent amber enclosure: leave the infill alone and just crank the wall count up instead. Four or five perimeters basically made the sides opaque and it barely cost more time than bumping infill did. On a map like yours the shallow bands are thin, so walls might cover most of the area anyway. Also curious what you're slicing this in, beacuse some of them let you assign a modifier block over just the ocean region. I did that once to force solid fill on one band of a print and it came out clean.

Another trick from back when we were all doing manual filament swaps on single extruders: put one solid layer between the dark bands and the translucent ones. I used to drop a thin opaque white layer right under any see-through color, acts like a backing and kills the see-through effect completley. Costs you a couple extra color changes but on a two-day map that's nothing. I did the same thing on a globe back in the day and the shallow water looked frosted instead of veiny, which honestly looked better anyway.

For the see-through problem, fuzzy skin saved a translucent teal sign I made last winter. Same vein issue, tried all the usual infill tricks and they only softened it. Turned on fuzzy skin for top surfaces only and the texture scatters the light so the walls underneath just disappear. It gives the surface a slightly sandy look up close, wich honestly suited an ocean map fine. Worth a test cube before committing to the full two-day run though.

Update: went with the extra walls idea and the sides look way better now, barely any streaks showing through. The shallow bands on top are still the problem though since they're only a couple layers thick, walls can't help there. The backing layer trick sounds promising. Does the layer under it have to be fully solid, or woud a dense sparse layer do the job?