[monochromatic]

Language evolves. Sounds like "on accident" is winning, and it'll probably take over completely in another decade or two.

But it'll always sound wrong to me.

[wmil]

I've never heard it in Canada. So it seems to be US only...

Actually most of the comments about it say that their children picked it up in elementary school. Given the wide and sudden spread I suspect that this can be traced back to some children's TV show or classroom video.