Q: What is the difference between the symbols used in ESD warning signs / labels: yellow hand w/ black background; black hand w/ yellow background; attention; triange/hand with arch; warning; etc. and how do you decide which one to use?
A: As per ANSI/ESD S8.1-1993, there are two types of ESD awareness symbols identified.
The ESD Susceptibility Symbol:
ESD Protective Symbol

ESD Protective symbol
Another common ESD symbol for safety would be;
Common Point Ground
On our website we have various ESD Labels, ESD Warning Signs and ESD Control Area Signs and Posters for your EPA (ESD Protected Area). The labels can be used on bags, boxes, and totes. The Caution Label symbol is still popular and is represented by a circle with three arrows entering into it.
The Attention Labels utilize the ESD Susceptibility Symbol and are represented by four different types:
ESD Control Area Signs and Posters.
Black on Yellow or Yellow on Black print is just used for ease of reading and the MIL 129 label is the only one that I know that uses a black hand in a yellow triangle instead of a yellow hand in a black triangle.