Hubble Palette

The tutorials I'm finding on Youtube to step through creating the Hubble Palette show working on a single layer RGB image and using a Selective Color layer for all color adjustments.  I'm confused because they don't show making individual channel color adjustments (also using Selective Color), which I used to find but can't seem to find anymore.  

My images are shot with narrowband filters and pasted into an RGB "bucket" with Ha, SII and OIII pasted into G, R and B channels respectively.  For example Nico Carver's tutorial called "Introduction to narrowband and Hubble Palette - 2a" makes one pass of Selective Color adjustments on the entire image.  That doesn't seem to make sense.  I thought each channel needed adjustments.  Can anyone straighten me out?  Thanks.