I have no way of testing this currently, but here's what the documentation says (emphasis mine) :
[-c|--color COLORTAG#rrggbb[aa]]
Override the default colors for the standard elements of the graph.
The COLORTAG is one of BACK background, CANVAS for the background of
the actual graph, SHADEA for the left and top border, SHADEB for the
right and bottom border, GRID, MGRID for the major grid, FONT for the
color of the font, AXIS for the axis of the graph, FRAME for the line
around the color spots, and finally ARROW for the arrow head pointing
up and forward. Each color is composed out of three hexadecimal
numbers specifying its rgb color component (00 is off, FF is maximum)
of red, green and blue. Optionally you may add another hexadecimal
number specifying the transparency (FF is solid). You may set this
option several times to alter multiple defaults.
What about making an almost transparent grid with arguments like these (note the extra 7F
parameter which translates to 127 in decimal):
-c MGRID#<hex triplet>7F -c GRID#<hex triplet>7F
It should still be visible in the background but be invisible (or barely noticeable) once any graph covers it.
Note that this answer from the developer of RRDTool says that the grid is always painted after the graph, so in the end you'll always have it in the foreground, your only solution is to either totally disable it or tinker with color/transparency parameters to make it invisible when covered by the graphed data.