Select by Color and Adjust

[psiclone]

I've already gotten past this problem in InkScape, but wanted to know how one goes about doing a selection by color (E.g. say you have an image with a white background) and making an image that has a blend of colors and gradient transparency be a solid color, while retaining the transparency.

 

MY PROCESS:

I had a heck of a time trying to figure out how to do this in both Affinity Designer and Photo, so that implies to me it could be more intuitive.  In Photoshop, I simply go to the menu and select by color, but instead of being limited to RGB (as in Affinity), or tonal selections, I could use a color picker to choose what I wanted on the image.  This would then select all of that color in the image and I could either mask or delete that.  I found the Transparency tool, but couldn't get it to work on the image and tried tonal selection which worked in general, but was much more difficult to figure out than simply picking the color I wanted (white).  I know part of this is simply figuring it out the first time, as I would make quicker work of that process the next time, but it's not always that easy messing with various settings instead of having a color picker.

 

THE POINT:

So, while a color picker on the selection menu would be a feature request, my question goes back to, OK, so I've selected the color I wanted as described above and now want to mask that.  I clicked on a mask layer and it worked, but now I want to invert the part of the layer that's not masked...which happened to be a blend of colors and transparency.  It was at this point I got stuck, as I couldn't figure out how to recolor the rest of the image to be one color, but still retain the gradient transparency it had.

[psiclone]

I know this wasn't necessarily intended for the latest update, but I'm still seeing a need for this.