I wouldn’t necessarily assume that; it seems entirely possible to me that Legoland model builders can get access to unreleased parts, or perhaps even request special recolours.
The reason I’m particularly skeptical that these are upcoming recolours has to do with the Mixel joints themselves. It’s been stated by Lego designers that, because the tolerances for these small-scale ball-and-socket joints have to be so tight, the molds are optimized for specific colours of plastic.
Of course, a Legoland display model doesn’t need the same type of quality control as publicly released parts, so they can get away with using potentially-out-of-tolerance recolours.