Nintendo has never revealed much about why Wario and Waluigi feel the need to copy Mario and Luigi, but there are plenty of signs that the duo are pretenders to the throne of great plumbing. Their unkept mustaches and parodic physical appearances are one clear indicator that the two are not to be taken seriously, but did you know that this philosophy extends to their clothing as well? And not just the inverse colors. Thanks to high resolution character artwork for Mario Party: Star Rush, observant folks at Tumblr have noticed key differences in the fabric used to make Mario's hat versus Wario's hat. It's some very interesting attention to detail.
The Mario Bros. and Wario Bros. hats are made out of completely different materials. While the Mario and Luigi have the embroidered emblems with sewn on borders, Wario and Waluigi have these felt-like patches, and it appears to be glued on instead as there is no stitches and the fabric is slightly raised. The even stranger detail is that their hats are a different texture to the Mario Bros. With their caps being a fuzzy material, while the original hats are made out of a more a cotton twill. Overall the Wario Bros. hats feel cheap and newer, which is very suiting.
You'd think that with all of the gold that Wario has greeded away over the years, he could afford to have a decent hat made for him. Nintendo's attention to detail is so important because it shows us smaller elements that contribute to the personalities and backstories of their characters. It would be easy to just apply the appropriate colors to the hats and call it a day, but not only did Nintendo's artists add textures that many people will never notice when they look at the artwork, they used the opportunity to choose appropriate textures for a minute detail like hat fabric and stitching. Or, as my professional tailor/seamstress girlfriend says, "Costume detail: gotta love it!"