we use bright colored painters tape.. yep the stuff that is kinda like masking tape only it comes in colors.. I know it is supposed to be easily removed and when i am ready to remove it it does come right off with out leaving any residue. But I normally leave it on there until it is ratty looking.. which is MANY trips.. We usually mark a BIG X on front and back of suit case.... and then run a strip around the outside edges... we never have a problem spotting out luggage. We get many grin and " good idea remarks" about it too.
I also have an older suitcase that is burgandy but appears black... I used craft paint and stencils to paint flowers and leaves and such on it... it too is easily spotted... but I think the tape is the easist!