Here's an adult Orange Shoulder Tang (Acanthurus olivaceus), also called Orangeband Surgeonfish or Olive Tang. We see them on the reef quite often. The juveniles of this fish can often be a bright yellow, developing the orange marking on the side and then darkening up over time. Here's a group of juveniles together. Oftentimes people will mistake them for yellow tangs, just remember yellow tangs are generally more roundish than oblong and you can tell the difference from a distance.
I'm not exactly sure why the color change exists, or if the juveniles are mimicking yellow tangs. In some species of fish, the juveniles will have a different coloration because the adults can be territorial and will beat on others that look the same - the small ones wouldn't stand a chance, so they have completely different appearance until they can handle themselves - - I'm not sure as this applies here, as the adults don't seem to be particularly territorial and are often found in groups.
We're back to the "slow" season again, not that spring break really panned out as crazy busy for anyone here, so no charter today or tomorrow for me. The last half of the week we're pretty busy though. I've got lots of charters set or started for later in the month, hopefully it'll be a reasonably busy April when it's all said and done.