The plucking position affects the high frequencies roughly like this:
- closer to the bridge : brighter
- closer to the neck : mellower
The author of this paper claims to have been able to write an algorithm that determines the plucking location of the string by listening to the sound:
https://www.ee.columbia.edu/~dpwe/papers/TraubS01-guitar.pdf
Quote:
2. PERCEPTUAL EFFECT OF THE PLUCKING POINT POSITION
Plucking a string close to the bridge produces a tone that is softer in volume, brighter and sharper. The sound is richer in high- frequency components. This happens when playing the guitar sul ponticello. The other extreme is playing sul tasto, near or over the fingerboard, closer to the midpoint of the string. In that case, the tone is louder, mellower, less rich in high frequency components.
In addition to the general bright/mellow aspect, the plucking point affects the set of harmonics generated. Quoting another page https://www.acs.psu.edu/drussell/Demos/string/Fixed.html
standing wave patterns which have a node at the pluck point will not be excited.
If an ideal string is plucked exactly one-Nh of the distance from one end, the resulting sound spectrum will lack harmonics that are multiples of N. But usually the plucking point isn't an exact multiple, and real strings are not ideal. See this question for more information on the harmonics: What determines the relative volumes of the harmonics when plucking a guitar string?