[GRASSLIST:257] r.mapcalc with TIFFs

[Peter Slizik]

Hi,

I have imported a tiff file (using r.in.tiff). I need to change some 
parts of this image, so I decided to create a mask and apply it to the 
original image using r.mapcalc.
After short playing with r.mapcalc, I realized it sometimes messes 
colors of the imported image.

For example, simple command

r.mapcalc
mapcalc> tmp = my_tiff + 10

just slightly changes colors.
But command

r.mapcalc
mapcalc> tmp = my_tiff + 30

totally messes colors of the image, making it look like a negative.
I assume it has something to do with the way how colors are encoded as 
categories.

Is there anybody who has more experience with this and could explain the 
mechanisms that GRASS uses or point me to some links on this subject?
Thanks in advance.

-- Peter