Set colormap of gTrack object, this is a named list of named character vectors that specifies the field of the underlying GRanges object that will be used to map a set of values to a set of colors.
usage: colormap(gt)[1] = list(tumortype = c(lung = 'red', pancreatic = 'blue', colon = 'purple'))
gTrack
to set the colormap for
New colormap