This error can be reproduced executing the attached Octave script.
The script configures a self-defined colormap, Then a so-called heat diagram is plotted on the computer screen, which shows four different color bars. Afterwards this diagram is saved (using indirectly gnuplot) to the disk as a PNG image. Using gnuplot 4.4 patchlevel 4 two of the color bars are different for the diagram on the screen and the image file saved on the disk. With gnuplot 4.4 patchlevel 0 the color bars of the diagram on the screnn as well of the image file saved on the disk are equal, which is as it should be.
This error occurs with gnuplot 4.4 patchlevel 4 which comes with Ubuntu 12.04.
This error does not occur with gnuplot 4.4 patchlevel 0 which comes with Ubuntu 10.10.
GNU Octave, version 3.2.4 which comes with Ubuntu 12.04 and 10.10 was used to plot the diagram.