If you have a Bambu Labs printer with an AMS then the plate file is included with the models pre-painted.

If you do not have a printer with multi-color capability, then you should change the filament about halfway through each panel in order to achieve the desired multi color effect.

Panels 1 and 4 would have the same color changes and the same could be said about 2 and 3. If you want to change up the color scheme, feel free. At the current sizing, you can fit 2 of these on a 256 x 256 mm plate easily. Please feel free to scale up the model and make a bigger wall art piece.

To achieve the red eyes on the Demon Asta file, it may be easier to just paint the eyes or use a red marker if you do not have access to a multi color printer.

0.2mm Layer Height is fine
No supports needed
As long as your bed adhesion is good then you shouldn't need a brim. If you have an issue with the print sticking then I would add a brim for added adhesion.
This is primarily for FDM 3D Printing, not tested with resin.