How to achieve a black and white conversion from color and then selectively apply tonal adjustments

[klj5]

Goal: I have a color image with pastels of very similar luminosity throughout.

1. Convert a color image to black and white using 'create new fill or adjustment layer' icon. 
2. Change color slider positions in new layer to achieve increased black and white contrast.
3. Select parts of the image that won't be affected by the contrast adjustment in step 2

Problem:

Step 3 is where I have my problem. eg. Having achieved the yellow and blue luminosity changes that I desire, I don't want the adjustment applied to all of the image - only parts of it. I want some of the adjustment to be applied 100%, some increasingly less than 100%, and some to have no luminosity adjustment. If I use a black brush on the white layer mask, it turns the painted portion back to color - not what I want. :>( If I create another bw adjustment layer on top of the 1st one, the color sliders have no effect. If I just duplicate the 1st bw adjustment layer and try to use a black brush, it has the same effect as if I'd painted on the lower layer. i.e. back to color. 

Please tell me how I can do what I'm trying to do - convert a color image to bw and adjust luminosty with color sliders SELECTIVELY. Thanx in advance for any help.

[Leslie Moak Murray]

The way to do this is by using channels. Many think they can just desaturate or convert to black & white in the adjustments  and leave it at that. Good for you for realizing there's more to it than that. Here is a good instrucional video on how to do it:

https://youtu.be/BLpAzo8yPdU 

[klj5]

Thanx for pointing me in the direction of the instructional video, ponytail. :>) I think my first Photoshop lesson was about 25 years ago, but I wasn't aware of the Image > Calculations until you pointed me toward it. There's quite a bit to it that I'll have to take some time to let it really sink in - particularly the fact that only 2 of the channels are being used - conceptually really very new for me. Thanx again.

[Chuck Uebele · Accepted solution]

I would most likely use at least two black and white adjustment layers. One to convert the image, and one or more below that one, and set their blend modes to luminosity. 

[klj5]

Thanx for trying to help me. I'm not clear, tho, on how the method you describe can be used to have a color slider adjustment apply to only a select part of an image - maybe, for example, all of the yellow slider adjustment to apply to the top left corner, 75% of it to apply to the top right corner, and none of the slider adjustment to the bottom 3rd all the way across the image's width, allowing no correction to fade gradually into the top two thirds as described in the 3rd point of my conundrum. I'd normally do this using layer masks and painting areas according to the result I want, but that doesn't work when color is changed to bw.

[Chuck Uebele · Accepted solution]

The top B&W adjustment layer is left on normal blend mode. It converts the image to B&W. You can add as many other B&W adjustment layers below that, and have their blend mode set to luminosity, so the image is still in color, below the top adjustment layer. You can then adjust each layer, then use the mask on each layer to apply the effect t to whatever part you want.

[klj5]

That worked!!! I've been trying to figure this out or get help from others with this for over a year now. THANKYOU!!