When I asked the pleasant young female teller why this was so, she explained (with a hint of embarrassment, if I'm not wrong) that the bank was promoting its car insurance 'green slips'.
While recognising the visual pun of 'green (balloon) plus (yellow) slip', I couldn't help but wonder why the slips were not green like the balloons. Perhaps it's a Dadaist promotion. If so, it's very clever.
I bit my lip. I know that I dare not pitch my wits against the superior intellect of a graduate team of marketing professionals from Head Office.