ps.map and colour tables?

[charnotw]

I have a raster which I have made by patching two imported sun rasterstogether.  When I display it on the monitor it looks great and the join is undetectable. When I use ps.map to get hard copies the join is very noticable; in greyscale or color mode the white on one side of the join has been rendered grey.  The raster is a map which I intend to use as a backdrop to a lot of output.  Does anybody know why this has happened? 
Is there anyway in Grass of reclassifying a map by its color table, so that all values that are represented by a particular set of percentages of red, green and blue are given one value?