The connected boxes show that the same color appears on both sides; however, the colored boxes provide context based on the lighting which is further reenforced by the rest of the dress outside the box.
Technology Connections did a video about brown that has the same color of orange on a background that cycles between white and black and the color of orange seems to shift to brown or orange depending on the background.
Sounds like a great chance to teach the child, they got to see the consequences of a bad decision directly and it will likely be memorable. Perhaps:
Also, maybe grab a suction cup toy, vinyl sticker, or other device to mark the highest the shower should be filled. A visual will help them know the limit.