In the original John Berkey painting, it appears that Vader is holding his lightsaber, stylized in a similar manner to Luke's lightsaber. (In fact, it almost looks like a solid metal blade with light glinting off it to me, but I think that's just stylization.)
Berkey's illustrations from early in the history of Star Wars were done with minimal access to information about the film as it was being made. As such, his Star Wars work is also notable for occasional inaccuracies. (For instance, one painting of the Death Star battle features multiple Millennium Falcon-like ships, since he didn't know that model was used as a unique ship in the film.)
The multicolored streaks below his hand seem to be just another stylistic flourish, typical of Berkey's painting. They may represent motion lines, as the color used is similar to the red-orange "contrails" left by the Y-Wings at the left of the image.