These are the guidelines for the ones DOING the baptism - the 'dunker' - and NOT the actual baptism candidates - as per Thisismein1972's question that I was replying to.
See the second sentence in paragraph 25.
I would take it to mean a white t-shirt (or may be white button shirt, short of long sleeved) and probably white shorts.
These guidelines - as can be seen from the instructions later in that same paragraph - are obviously for a pool baptism when everyone is in actual the water - NOT for a cattle trough style baptism.
Guidelines for the actual baptism guidelines are along the following lines
KIngdom Ministry June 1993, page 3
Question Box: What would be considered proper attire when presenting oneself for baptism?