While some have pointed to this being a part of the Zack Snyder aesthetic (he does love his muted colors), there is a more natural explanation.
As has been pointed out by fans, Wonder Woman’s costume also looks just a bit different than the one we saw weeks ago in her solo movie — specifically in that it looks a good deal less colorful.
The good news is that she still looks like a total badass… but one does hope that she find a Leathersmith at some point to bring the boldness back out of the costume.
Namely, Wonder Woman is probably still using the same armor that she took when she left Themyscira, and it makes sense that the strong reds and blues would fade after a century.