Polarr is a comprehensive editing app with an interesting pricing mechanism. You can use the app for free but with some limited functionality or pay for the 'pro' version. The $20 for the full version though also gives you access to the desktop versions.

Features (from Google Play)

  • Complete set of color masking and brushing tools
  • Create, customize, and share your own filters
  • Multiple local and brush adjustments
  • Customizable toolbars and workspace

Adjustments include:

  • Color: Temperature, Tint, Vibrance, Saturation
  • Light: Dehaze, Exposure, Brightness, Contrast, Diffuse
  • Detail: Clarity, Sharpen, Denoise (Color and Luminance)
  • Vignette: Amount, Highlights, Roundness
  • Lens: Distortion, Horizontal and Vertical Perspective
  • Effect: Fringing, Pixelate, Noise amount and size
  • HSL: Hue, Saturation, Luminance for eight color channels
  • Curves: Master, Blue, Red, Green channels
  • Toning: Highlight and Shadow tone, Tone balancing

I like the design of Polarr. It offers a wide range of tools but the interface and icons are relatively intuitive and easy to master, especially with the built-in tutorials. It is a little slow to start though and occasionally temperamental, especially compared to Snapseed for example.

Best used for:

  • Curves adjustments
  • Brush and filter tools
  • Range of filters