With regard to blind perfs - I know nothing about printing, but I believe that they are created by insufficient pressure to cut completely through the paper.
Similarly, light or heavy inking on a stamp can be caused by incorrect fluid flow settings, ink reservoirs that are and incorrect pressure as well. All can result in a stamp that does not have perfect colouring.
I have never considered a less than perfectly coloured stamp an error or a variety, just interesting and yes, I do collect them.
Of the following four images, I have no idea which, in any, have the correct colours.
