Hiya my aunt does all this kind of thing and she always stamped the outline with a darkish colur it does not have to be black but you do want it to show up and then once dry coloured them in, she has done ones with chalk but you have to mind they don't get too smudgy, felt tip, wax crayon and watercolour paint if you are really arty.
Most of the stamps sold now are just the rubber bit so you will need to by at least one acrylic block a little bigger than the largest stamp you buy, you just dampen them a teeny bit and they self adhere then you just peel it off when you want to change it for the next one.
Another tip is to put the stamp face up on the table and gently tap the ink onto it so it only goes on the raised area and practise on some cheap paper until you get the pressure just right as some card or prebought cards can run expensive if you don't get it right a few times.
Good luck with it and i hope they come out wonderfully

Sam