There are a couple of possible reasons, actually. For one thing, it could simply be genetic; if your volcarona’s parents were pink or otherwise not as vibrantly red as standard, then that increases the likelihood that they would have offspring that would be pink or off-color in their adult stages. Supposedly, a shiny parent also increases the likelihood that something like this will happen, but frankly, this hasn’t really been confirmed.
Second, it could be diet. Sometimes, eating pale or pink fruits in one’s larval stage changes one’s color later on. This phenomenon is especially true for pokémon that feed primarily on pinkan berries (which are notorious for changing a pokémon’s color after just a few servings), but sufficient amounts of pecha or persim will do it too.
Finally, it could also be environmental. If you trained your larvesta mostly during the evenings or early mornings, particularly during sunrise or sunset, there’s a chance that it will adjust its colors to match those of the sky at that time. Otherwise, most larvesta will try to match the warmth and vibrance the midday sun (even if it’s by memory, if they’re mostly trained at night).
In other words, the pokémon center was absolutely right. Your volcarona is perfectly healthy … just fascinatingly rare.
Congratulations on both this and their evolution!