How to make dark gray background on TIF image transparent or white in QGIS

see also: Imported TIF has dark gray background instead of white

That question addresses why the TIF imported into QGIS has a dark gray background when the color set is set to 4096 when exported from PDFCreator. That file's gdalinfo shows NBITS=4

Here, in this question, I would like to know if there is a way to render the dark gray transparent, much in the way that entering a "0" in properties->transparency->no data value->additional no data value will make the black lines transparent.

Alternatively, is there a way to have QGIS render the dark gray as white?

I'm thinking, without much background in this, that adding custom color band transparency values will do the trick, but I have no way of gauging what the band values are for the dark gray

I tried looking at a histogram, and it drove a peak on the blue at 11 or so, and entering that value didn't help on the additional no data value, and I couldn't figure out what to enter on the custom boxes.

Ideas?