Watercolour pencils are easy to use and don't make a mess. You simply dip your pencil in the water and then smudge along your paper until you achieve the desired effect.
The teacup cards are available as a free printable from Creative Comforts blog (along with a lot of other lovely things)
I took one of these and attached it to a blank card, along with a few little embellishments. They would also make pretty shower (baby, wedding) or tea party invitations, don't you think?