plot() function: color transparency

I found all the color transparency was defined with character color, or rgb color. What if I have number code and still try to modify the transparency?

For example:

x=c(1:5)
color=c(2,2,3,4,5)
plot(x, col=color)
plot(x, col=color,pch=20)

here I defined color by numbers, how can I modify the transparency?