With the primary color set to transparent, the "sampling once" mode does nothing, with either the left or right mouse button when clicking anywhere in the white portion of the png image. The color that I'm trying to make transparent is white. Previously I was able to set transparency with the bucket and the portion of the image which was changed was displayed with a checked background, but replacement was a little choppy so I thought that recolor tool might be more precise method. There is zero change in the image display using the recolor tool. It may be performing a one pixel-per-click replacement which isn't large enough to make any of the checkered pattern to show up. The reason that I say that is because when I use the tool with 2 visible colors (alpha equals 255) it only recolors one pixel per click. I don't think that this is the expected behavior as the documentation states "The Recolor Tool is used to replace regions of similar color with another color.", and I had used it to replace red with green once and the recoloring was global in the image with one click (same PDN version). My image is four large ASCII characters on a white background, and saved in png format (using the windows 10 snipping tool).
It could be that the tool does not support recoloring with transparency, but that doesn't explain the one pixel per click change that I'm seeing with "visible" colors. I'd be very appreciative to anyone that could help me to get to the root cause of this issue. My time is limited on this, so I would have to move on to using a different program. Thanks in advance to anyone who can help.