Please see the style guide here: https://design.wikimedia.org/style-guide/visual-style_colors.html#utility-colors
Currently notifications with type "error" look like this;
But could look like this after following utility colors from the the style guide.
Which also matches with the OOUI MessageWidget with type "error".
The same applies to notifications with type "warn" compared to the OOUI MessageWidget with type "warning".