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?

Thanks a lot for any input!

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