Could someone explain how Blender output colors I'm newbie in the Blender, and can't understand why colors are adjusted during output.

Here is mine compositor scheme


As you can see, the R channel equals to 0.4 in the Combine RGBA Node, but in output it's 170 of 255, ie 0.666. Why does this happen?

Have to mention, it is purely 170 all over the rendered image, so it's not a compression artifact.
Well, solved ! :)

Thanks to the user @cegaton !
Here is what he wrote to comment 1st answer:


  @m.ardito The important information you left out
  of the picture. What you are showing are the scene referred values in
  32bit float:The compositor works with scene referred values: unbound
  linear data. Data is then processed through a display transform
  (determined in the Color Management section) and converted to Display
  referred data (in a scale from 0 to 1) The default transform is to
  sRGB and is not a linear transform. That is true for images saved in 8
  bits or 16 bit. The only format that gets saved bypassing the display
  transform is OpenEXR


So after i set Properties editor ‣ Scene ‣ Color ‣ Display Device to none, all looks correct. Value 102 of 255 is 0.4.

Thank for the help you both @cegaton and @m.ardito !