ggplot(data.frame(x=1:7),aes(x=1,y=x,fill=x)) +
 geom_raster() +
 scale_fill_gradientn(colours=rainbow(7))+
 theme_bw()+theme(legend.position="none")+
labs(x=NULL,y=NULL)+ theme(line = element_blank(), text = element_blank())

Courtesy dj patil