The cbrange command sets the range of values which are colored using the current palette by styles pm3d, `with image` and palette. Values outside of the color range use color of the nearest extreme.
If the cb-axis is autoscaled in `splot`, then the colorbox range is taken from zrange. Points drawn in `splot ... pm3d|palette` can be filtered by using different zrange and cbrange.
Please see xrange for details on cbrange syntax. See also palette and `set colorbox`.