I find that the Hong Kong Dollars can be quite confusing for 1st timer like me.
Take these HKD100 for example, there are four different version here; they might be other versions but these are the only HKD100 that I brought back with me.
For HKD10, that is even worst, the colors are slightly different from one another.
I don't use much cash during my 5 days there, most of the shopping I just use credit card. I only pay cash on food and street shopping in Mongkok.
For credit card, the bank informed me that they will charge extra 1% percent for usage oversea; but I haven't check my bills yet.