Darktable is a bit laggy (sadly), but it has a whole bunch of
amazing filters that not even the latest Photoshop or Lightroom has, so it is well worth diving into. The equalizer tool is incredibly powerful and flexible, for example[0].
That linked video is by Robert Hutton, he makes really good tutorials for Darktable so that might help you.
Anyway, for non-realistic stuff you can add G'MIC for more filter-options and you are pretty much set for life[1].
[0] https://www.youtube.com/watch?v=zzVXK4eAM5E
[1] http://gmic.eu/