The reference point for the healing brush was far from the collar. But as the application of the brush got near to but did not overlap the collar (according to the representation of the brush on screen), the result was darkened -- and darkened far too much. The skin tone I was healing had quite a significant element of black introduced into it.
This is a bit of a bug, I think.
Not really, this is just unfamiliarity with how they work. Every time you start painting again with the brush, each new left-mouse click, it starts again from the origin you set. If you drag the brush far enough during one brush stroke it will copy those textures relative to your origin that you set. You just dragged the brush too far without letting up and clicking again to reset the painting data from the origin.
Something I particularly like about the healing brush is that when you set the reference point, it is absolute. You move the brush but the reference point stays the same. Great.
But with the duplicating brush -- the healing brush's brother -- the reference point is relative. You set it, then when you apply it, you set the relative position. Move the duplicating brush and the reference point moves in parallel so you have to keep an eye on it all the time when you are copying a number of times.
I would like to see this to be fixed by default, as the healing brush reference point is so that I could be sure that I am duplicating.
Perhaps both could benefit by having an absolute point of reference as a default, with an option for it to be relative.
Look on your Tool Settings dialog for the clone brush, there is the option to check for "keep distance". This moves the origin relative to the brush, keeping the same distance. Uncheck it and it behaves like the healing brush.
Since the repairing (healing) brush is most often used to copy a texture from just one small area, and applying it a larger area or vastly different areas (say in spotting out skin blemishes all over a face), there's no need to have the relative "keep distance" option for it. Say you wanted to clone some fur texture from your fuzzy doggy to your hairless chihuahua. You click the origin as fur on the fuzzy doggy, then brush that around the places you want more fur texture on the hairless one, with small strokes in individual clicks so you are always resampling from the origin no matter how far you move the brush. If you move the brush too far during a single brush-stroke you're off the dog that you sampled the fur from. The "bug" that you had happen above would happen all the time if there was a relative "keep distance" option for it. It's better to have the origin absolute on the healing brush with each new click. If you do need to have it act as relative origin, then just don't let up on the mouse-click and it'll follow along the length of your brush stroke.
The reason a "keep distance" relative-origin is always an option in clone brushes is that many many times you have to clone out a geometrically straight item that runs a long distance in an image, or rebuild a long edge from one that's not damaged, all aspects of the image need to be copied, not just the texture and details (as in the repair brush). Objects like a fence-rail or telephone line, sampling right alongside it while overwriting with similar image data in the same area, the nearby background data which will change drastically along the whole length of the object that you are cloning out. Many times applying the same data in several brush stokes that can be duplicated to apply it with a little more intensity each time. I've never run into a situation where that's needed for any "healing brush" type of work.
If you absolutely must have a relative-origin healing brush (that's not as effective), you can use the Channels option in the Tool Settings dialog for the clone brush, select the Lab or HIS channels, then DE
select the "ab" channels if in Lab mode, or the HS channels from the HIS mode. Then you will only copy the "L
uminance" or "I
ntensity" from the clone/copy-brush's moving relative-origin.
You just need more practice with painting tools. I didn't realize when replying to you earlier about the skintones that you were more into DTP and had so very little experience with actual photo editing tools. This explains the incongruencies of importance we each find in these tools. I use the DTP options of PL32 so rarely that I don't even know most of their more intricate workings, I even forget that people use it for that. Gerhard even had to explain to me why I was getting some weird border around my photos using one tool, because I totally forgot about (didn't know about) setting some default "document" size options.
I agree that a preview-brush mode (like the clone brush has) would be nice for the healing-brush (repair brush), but because of how healing/repair brushes work, having to work off of an intermediate mask (see the mask color as you paint with it?) and do several other complex cacluations between origin and result, I don't think a preview-brush mode is possible for it. I've asked in the past if it could be done and it wasn't addressed in replies to other questions I had at the time, probably because I had already answered my own question when asking it (referencing how it works, as I just did again).