When you mate two adult characters, it will 'mix' their traits together. This can result in some... interesting combinations, and the traits will last throughout the generations.
You can influence what traits are more likely to be passed down by changing the town's theme. It seems to be influenced by what you do with the parents, but only seems to change once the new baby is born and goes to the town. You do this by feeding them food, playing with items, and dressing in accessories from the area the parent whose traits you want came from. I'm not sure how many themes there are, and it changes depending on what version of the m!x you're playing with.
The m!x is an extremely lovely Tamagotchi, particularly the 20th Anniversary versions. The coloring behind the faceplate is starry and metallic. It's just really, really nice to look at. Dust doesn't seem to get behind the faceplate, either, which was a big problem I had with the Tamagotchi P's.
The screen is big and bright, and the changing of the menu means the characters have way more room to move around.
It's really, really cute to see the characters moving around the screen and interacting with each other, and I love being able to mix traits of my favorite characters.
If you want a color Tamagotchi, I'd really suggest taking a look at this version
Tamagotchi m!x - Anniversary Version - Part One