Calculations for proportional increase in grounding wire size are right on the cusp of #6 and #4. Depending on whether I use mm2 or Cmils, NEC Table 8-Chapter 9 suggests different sizes.

You don't need to work that hard. AWG sizes are on a log/exponential curve, which makes proportions stupid easy. You compute the minimum legal wire sizes. If there is a 2 AWG size difference between them... then when you upscale wires, simply maintain that 2 AWG size difference.

If you don't understand why this works, look into how slide rules work.

The distance between panels is 265 feet.

Well, right off the bat, you don't need to compute voltage drop based on breaker trip. That is foolish and wasteful, since voltage drop can't read :) Even it could read, it can't see the number on the breaker handle from its vantage inside the conduit LOL. You compute voltage drop based on the actual loads. Or at worst, 80% of voltage drop because you shouldn't plan to load a circuit more than that.

and a 60 amp main breaker in the subpanel (rated 125 amps)

The main breaker in the sub is not necessary unless you have more than 6 hand throws in the subpanel. Note that 120V circuits can be handle-tied 2-3 at a time using approved handle-ties, making them count as 1 hand throw.

If you insist on having it, remember a tie-down kit is mandatory.

What size bare, solid copper wire for EGC running to ground rods from sub? I thought I read that this is sized off breaker not feeder conductors. I also read this needs to be equivalently sized as the EGC running back to the main. It’s only running about 12’ out to ground rods.

Pump house? For a well? Gosh, it sure would be nice if there was like a 300-foot-long steel thing going straight down into the ground, right there... that would make a pretty good ground rod!

Color coding just the terminal ends of the grounded conductor with white tape is code compliant as long it is at least 4 AWG, right?

If by "grounded conductor" you mean neutral, yes. I don't recommend internalizing the NEC lawyer-speak for that word, it's too darned confusing.

Calculations for proportional increase in grounding wire size are right on the cusp of #6 and #4. Depending on whether I use mm2 or Cmils, NEC Table 8-Chapter 9 suggests different sizes.

You don't need to work that hard. AWG sizes are on a log/exponential curve, which makes proportions stupid easy. You compute the minimum legal wire sizes. If there is a 2 AWG size difference between them... then when you upscale wires, simply maintain that 2 AWG size difference.

If you don't understand why this works, look into how slide rules work.

The distance between panels is 265 feet.

Well, right off the bat, you don't need to compute voltage drop based on breaker trip. That is foolish and wasteful, since voltage drop can't read :) Even it could read, it can't see the number on the breaker handle from its vantage inside the conduit LOL. You compute voltage drop based on the actual loads. Or at worst, 80% of breaker trip because you shouldn't plan to load a circuit more than that.

and a 60 amp main breaker in the subpanel (rated 125 amps)

The main breaker in the sub is not necessary unless you have more than 6 hand throws in the subpanel.

Note that 120V circuits can be handle-tied 2-3 at a time using approved handle-ties, making them count as 1 hand throw.

If you insist on having it, remember a tie-down kit is mandatory.

What size bare, solid copper wire for EGC running to ground rods from sub? I thought I read that this is sized off breaker not feeder conductors. I also read this needs to be equivalently sized as the EGC running back to the main. It’s only running about 12’ out to ground rods.

Pump house? For a well? Gosh, it sure would be nice if there was like a 300-foot-long steel thing going straight down into the ground, right there... that would make a pretty good ground rod!

And yeah, you're entitled to use that if it's metallic. Thanks ThreePhaseEel.

Color coding just the terminal ends of the grounded conductor with white tape is code compliant as long it is at least 4 AWG, right?

If by "grounded conductor" you mean neutral, yes.

I don't recommend internalizing the NEC lawyer-speak for that word, it's too darned confusing. Someone should issue a Zach Snyder's NEC with neutral called neutral. It's what King James would do.