brighten the current palette by the specified FACTOR.
The FACTOR is the slope of the transfer function for the color value
(see to_hsv for a description of the hsv color system); a value of
1.0 always remains 1.0, but values near 0.0 change by FACTOR.
FACTOR= 1.0 is a no-op. The default factor is 4.0.
interpreted function, defined at i/color.i line 38