I am trying to set a solid ( non 4 colour ) black in Photoshop. I am able to do this successfully; however if for instance I create a shape or a piece of text using my 100%K black all is well until I go back in to check the fill colour and then into the colour picker ... and without touching anything in there the colour has changed back into a 4 colour black. I cant see any way to make it stay as 0 0 0 100. I have been using this video as a reference and I can see this is is definitely possible, but I can't see what I am doing wrong. Is there some setting I'm unaware of. I'm using CS6.
Your document needs to be in CMYK. To validate this in Photoshop for your document, go to Image > Mode > CMYK Color
Now select the piece of text and check you've got solid black (0,0,0,100) in the Select text color window, see image below:
I had same exact problem and took me forever to figure it out. In the color picker as the other person showed here, create your 100% black color and save as a swatch. Then highlight your text or shape and then click the swatch (for text), or fill it for a shape. It should be correct when you check with eye-dropper now - BUT, if there's any transparency to your element and if there's color underneath, it obviously won't appear as 100% black with your picker (took me a few minutes to realize that).