When Ben destroyed the original Omnitrix, he rushed to steal the Ultimatrix. At first, it had a red icon, signaling default or unstable settings.
As soon as he got his hands on it, Ben manually changed the color to green, just like he once unlocked Master Control on the Omnitrix as a kid.
Turning the dial on the Omnitrix and Ultimatrix allows color swapping, giving Ben control over its appearance.
Much like today’s computers have customizable interfaces, the Ultimatrix has a range of color settings, with green being Ben’s personal favorite!
At the end of the day, the green glow is iconic—just like Ben himself!