According to the whole market nowadays.
There are 3 main STN glasses: STN-gray, STN-blue,STN-YG
And 2 FSTN glases: FSTN-pos, FSTN-Neg
It's more easier when the backlight is on.(white)
STN-gray+ white bl= blue font with white background
STN-blue + white bl= white font with blue background
STN-YG+ YG bl=black font with YG backgroudn
FSNT-pso+ white=withe font with black background
FSTN-neg+white=black font with white background
The information from Topway_stanly is accurate, here is the "cliff note's" version.
FSTN is essentially "black" for negative mode and "white" for positive mode -- both will give you better contrast than STN. STN is good if you need some color in the background such as blue or yellow green.
They are both good, standard LCD technologies, and it really boils down to the look you want. For example, if you want to have white characters on a blue background, you would go with a white LED backlight and STN Blue LCD, our "TMI" variant. If you want a darker background then you would opt for a FSTN Black LCD and white LED backlight, our "TTI" variant.