Brush won't paint on just the selected layer

Love  your question... I  not sure of what is actually happening, but I can offer related issues which may play a part. .

First of all, I don't think it was the intensity of color.  A layer below one above, cannot bleed into it by virtue of how white it is.  What it could be, is your layer is not set to full opacity.  Then, if an above layer is at a low opacity, then yes, you can affect it by virtue off it has a high degree of transparency.  You choose your layer, then look closely you will see a slider saying 100 percent or whatever it is set on. As you move it, to a high number of lower, you will see the result in real time.  Also, you look at the type of layers. You see a drop down list with titled layers as: Normal, Dissolve, Color Burn, Linear Burn, Pin Light,  Soft Light, Hard Light, Luminance, and more.  Even as you have a layer painted, click one of the layer types in the drop down menu.  You may find it very interesting.

oops, one more common mistake, is if you make a selection, then get up like to give me coffee, at times when a person has left the chair for a moment, then tries to paint in the image they created, nothing occurs. If you painted inside the selection, you cannot use any other part of the image, until you de - select, or you can choose 'inverse. '  Inverse is a choice of what may have been protected by a layer mask, or not protected, then needed.

What you should also check, (a very small item but can simulate a serious bad)  Is your select or delete menu.  I have seen people swear photo shop has crashed, because all menu and brushes are not  reacting by choice or selection. No color or paint is responding at all.  What happens to create that condition is so small. lol, when you make a selection with one of the tools, you see the black sort of dotted in motion lines.  That is the outer line for your drawing or coloring in.  Some tools can leave behind a fraction of the dotted line. very small, and often not noticeable/   ...    the dots i just typed are enough to affect what I am describing. If you had made a selection, and suddenly you are with no active tools, that the selection ants have not all been cleared away.  As stated only a couple you may  not see, that are left behind.will render Photoshop dead in the water seemingly serious.  But it is only that until you leave that selection, the program cannot advance.   lol,  When i began to learn Photoshop, myself and a friend agonized for an hour before calling a pro support.  The tech asked what she was doing, and if any selection had been made.  A very large omg followed.  She found a couple of dots,left behind, so until she removed  them, the program was frozen.

Example, if you color a photo, and need to paint over lips, if you use normal, it may be very difficult to get a good result. l.  The color is too opaque. So one may choose to increase the transparency so the lips below the dedicated layer color layer have a more natural texture and we see them easier. But that may NOT be the best route.   On the layer drop down list, check the 'Color' type, you  will find the painted  layer on top of lips, will be a colored, but clear also showing the beauty of the lips in the layer below.  You should choose you pic or part of one, and experiment with the layer types.  lol, then if you choose one, and choose to 'Fade' what ever tool is used, you can control so much of the color intensity,  texture and so many variables. It can really assist a person who is in a early  stage of  knowing photoshop.

I hope this has some merit for you.

Best,

Z

Hey, that's a clever trick :-)

Grab a screen shot of your entire workspace showing the image window and the Layers panel, with the layer you are trying to paint to selected.   Just copy to the clipboard, and paste to this thread with Ctrl v