Agree w/Jhong - although some of the test chemistries are the same FW and SW, the color scales may be quite different, so the readings may not be accurate in BW to SW if FW color charts are used.
I agree with nearly everyone in this thread but thought I should toss in the following - which test kit you use and which chart you read will depend on the SG of your water. When it's low BW (1.008 or less), I'd opt for the FW test kits and charts, but as you make the water more brackish I'd switch to the SW kits.