Trainers cannot have shinies unless they're programmed to have shinies. An infamous example is in the Battle Tower in FR/LG, where I believe some trainers are set to have a shiny Espeon or Seaking.
Wally's Ralts isn't set-in-stone, so it may be shiny. In addition, the wild pokemon in the Battle areas (such as the Battle Pyramid or Battle Factory) are randomly generated and could be shiny.
Either way, if it doesn't have a special star animation, it's not shiny. Don't rely on the color all of the time. This reminds me how some confuse the two different colored Shellos, lol.. And there are other sites to view sprites besides Serebii.