Palette type.
There are two available options:
"default" (51-color palette);
"alternating" (2-color palette).
alpha
Transparency level, a real number in (0, 1].
See alpha in rgb for details.
References
James T. Robinson, Helga Thorvaldsd\'ottir, Wendy Winckler,
Mitchell Guttman, Eric S. Lander, Gad Getz, Jill P. Mesirov.
Integrative Genomics Viewer. Nature Biotechnology 29, 24--26 (2011).