The question is purely subjective
"what is specific range of contrast for vegtation,waterbodies"
Contrast consider as the dynamic range of pixel values, if the image having all pixel values (within it range), then it is consider as a good contrast image (see the histogram profile) I dont think constrast may consider specificall based on things (vegtation,waterbodies).
Are you looking for segmentaion for vegtation,waterbodies from the image.
"how to assign colours to different range of values of (say contrast) to classify into vegetation,waterbodies etc"
Better you should look for color levels or composition (not contrast), or color thresholding