@Voronwe13 Yeah, I think hypothetically the two options should be available since the first one reproduces a already “standard” workflow for many users. It’s after all the technique @Mythmaker used on his awesome piece.
For me the use case of the second option - sample RBGA information - would be to reproduce the randomness of a real brush subjected to real physics loaded with real mixed paint.
The objective of the “mixer brush technique” is always to diversify your mark making, a fancy way to say that you should have interesting diverse textures throughout any piece. Artist would constantly be sampling - even in the standard mixer brush technique - so that every other brush stroke looks internally different from the previous ones, even if they are trying to keep the same color. If the RGBA information is locked to the brush that’s already a big improvement from what we have, but if it could be changed in a almost per stroke basis, by constantly sampling the canvas, that would be very close - in appearance at least - to dealing with real oil/acrylic paint.
If I may be so bold as to suggest workflow at this early stage:
Ctlr = Color Sample / Clone sample if the functionality is available for the currently selected brush
Ctrl+Alt = Color sample all layers
Ctrl+Shift = RGBA infromation Sample
My only concern if that goes into the smudge brush engine is that it would basically kill the clone brush engine. And if the clone brush engine goes away it won’t be as intuitive for some new users to look for that function under the Color Smudge Engine.
PS: The dream function: “Ctrl+Shift” will sample a dab worth of RGBA information. Alternatively if the user holds “Ctlr+Shift” right clicks and drags it across the canvas the initial point of contact with the canvas would be the center and the final drag point would be edge of the drawn radius of a “RGBA sample circle” from which the brush engine would randomly sample a dab’s worth of RGBA information on a per stroke basis. That would streamline the painting process - and complicate the coding process 
- and make it for much more interesting stroke basis.
If that last suggestion was too convoluted - which I think it was - I can make a mock up for you.