transparency using plot+points with sp classes

How can I make one point graphics with transparency

These are all sp classes:

plot(polygons_area,axes=TRUE,asp=1.5,main="Title",xlab="Latitude",
ylab="Longitude")
points(observations2000,type = "p",pch=21,col="green")
points(observation1999,type = "p",pch=21,col="blue")
points(reference.points,type = "p",pch=21,col="red")

how can I add transparency to one of the point graphics, for example
points(observation1999,type = "p",pch=21,col="blue")  ?