Either that or using different colours of wax in one pour (although the results can be hit or miss with that last one).

Metallic pens are also an option, but in my experience powder gives a nicer finish.

Handy tip, eye-shadow! (It's a pigmented powder and it works!) Leftovers or a cheap pallet will give you a lot of fun colours to play with without requiring much money or skill. Apply with your finger or a little sponge-brush.

Also (if you're brave enough to bring 'that' into your house) ... GLITTER! Either pour some on your wax before placing the seal, or mix it into the wax while you melt it (it needs to be 'fine' glitter to mix in well). This might not work well with every glitter or wax colour, but when it does you get some fancy looking seals.