One thing you can do to pull out only the chroma like that in post is duplicate the background layer then go into the separate color channels. Adjust the levels

One thing you can do to pull out only the chroma like that in post is duplicate the background layer then go into the separate color channels. Adjust the levels on just the channel you want until you've got the definition of the shape you want. Wand select the white background and then invert your selection. Undo the level changes and go back to the composed duplicate layer and create a Reveal mask from your selection. Everything outside the selection will be knocked out. Then you can adjust the levels on the composed image to get your desired brightness/color/contrast. Do the same for other output color layers like the yellow in this example. For the Luma convert the color space to HSL and copy the Luma layer then adjust to your desired definition. This lets you create your "plates" for each separate color. Using masks instead of blend modes on the layers will get you the plate separation effect you want. Once you've got your "plates" knocked out apply the mask then you can add all your other effects on the layer like the halftone. Switching color modes and level adjusting is also a good way to do knock outs with the want selection. For instance doing a selection and getting wispy components like strands of hair. Look through the color channels where there's the best definition and adjust levels to bring out the desired contrast. Select that and then apply it as a mask to the composed RGB layer. You can then paint into the mask to knock out extraneous bits outside the bounds of the subject. Modern wand select tools have a lot of little features like adjustable sensitivity and sometimes can create masks directly. But the above method tends to work well in software with Photoshop 5 quality tools. It's a bit easier to do all the work in post than with the camera. Obviously if you want to do the work optically ignore the above.