Dip each coffee filter into one of the coloured water bowls for about 10 seconds, letting the colour absorb. You can dip the lower half in one colour and the top in a second colour for a gradient. Gently squeeze out excess liquid and lay each filter flat on a sheet of kitchen paper. They will look much darker wet than they will dry.
Leave the filters to dry completely on the kitchen paper. This takes at least 1 hour 30 minutes at room temperature. Do not rush it with a hair dryer: the filters wrinkle and the shape is ruined.
Once dry, scrunch the centre of each filter from below to form a round head shape at the top, letting the filter skirt fall open below. Twist the neck gently to hold the shape.
Tie a length of twine around the twisted neck, leaving a 20-25 cm tail above for hanging. Use the black marker to draw two small eyes on the rounded head.
Tie the hanging twine of each ghost to the twig at even intervals, varying the lengths slightly so the ghosts hang at different heights. Knot firmly. The twig mobile can be hung from the ceiling with a single central loop of twine.