Summarize:

There are many different types of sealant you can use. It depends on how aesthetically concerned you are about your smoke bombs. The simplest method is to apply a small strip of tape over the opening. You could also seal the hole by using tissue paper and a glue stick. Cut the tissue paper a little larger than the shell's opening. Use standard tissue paper like you'd use to wrap a birthday present. You can use different colors of tissue paper if you want. You can take this in any direction you want. If you'd like a simpler design, draw something with a marker. You could also paint a fun design using watercolors. Remember you're going to ultimately break this, so don't become too invested in your design. Be careful when decorating; the egg's shell is still very fragile. This step is optional.
Gather the sealant. Cut tissue paper. Decorate your smoke bomb.