we have seen the green colour in orion nebula in a 10" dob too! just a hint of colour though.. i understand you need to go up in aperture to start getting more. i think people say 'no colour' to prepare people who might otherwise be expecting magazine photo style colour
my guess at why stars have colour but generally not other objects is that the stars are brighter? more photons to make the cones in your eyeballs work?
the factor of the barlow depends on your eyepiece selection... as the magnification gets higher, the ability to use it depends more on the conditions. so your 21mm eyepiece, with a 3x barlow acts like a 7mm... but a 2x makes it 10.5mm - that might make the difference between usable and not, depending on the seeing any particular night. plan your eyepieces carefully to get the best value from your $
i dont think there is a technical reason not to get a 3x.
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