Consider using different colors for `error` and `warning` icons

[stefanobartoletti]

First of all thanks for this tool, it makes configuring eslint really easy.

I have a little suggestion though, as the title says using default colors for error and warning in the icons.

Right now both the icons are of a slightly different shade of green, but they look very similar, so it is not that easy to tell them apart at a glance.

Furthermore, the generally accepted semantic colors for warning and error (not only in computer programming) are respectively yellow and red.

Especially since green also has a semantic value of "Everything is ok", so it visually negates any error or warning.

I think that this small fix would greatly help in understanding the output and reduce cognitive fatigue.

Thanks!