Thanks for the reply, Ashley. Definitely sounds more complex than I thought it might be.
And R0J0, wowww this pixelate effect is a game changer!! Even ignoring drawing canvas for a moment, just throwing pixelate onto a sprite and setting pixel size to 1, and then using "set colour", works fine with linear filtering!! This opens so many doors for me!
The only issue being when zoomed out far away with 3DCamera, the set colour struggles to apply (I guess the pixelate effect is struggling to apply too), and another issue with pixelate effect not applying at all on non-square images, but these are minor issues (and might be related to scaling which I need to wrap my head around a bit more). Beyond this, you could bake the colours/changes to a sprite by utilising drawing canvas to paste the clean pixelated sprite, make changes to the drawing canvas, load back into a sprite via Load URL, then there's no need to worry about the effects applying correctly anymore. Amazing!!
I always thought the pixelatte effect was more for smaller little effects or screen transitions or something, but now I perceive it differently and see it as a tool to allow you to use linear filtering in your project, whilst still allowing you to choose individual objects that you would like to have point sampling applied to.
Many thanks for unlocking many new ideas in my mind, and thanks for the example c3p. I hadn't explored the "using mouse to draw on canvas" aspect yet, so this helped a huge amount!!