Unlike partial opacity, translucent colors don’t make content disappear from the print output in FF4b9 on OS X 10.6.6.
Similar test with opacity instead of RGBA colors.
A paragraph with color:rgba(0,0,0,1)
A paragraph with color:rgba(0,0,0,.99999999)
A paragraph with color:rgba(0,0,0,.99)
A paragraph with color:rgba(0,0,0,.9)
A paragraph with color:rgba(0,0,0,.75)
A paragraph with color:rgba(0,0,0,.5)
A paragraph with color:rgba(0,0,0,.4)
A paragraph with color:rgba(0,0,0,.25)
A paragraph with color:rgba(0,0,0,.1)
A paragraph with color:rgba(0,0,0,0)
A paragraph with color:rgba(0,0,0,1)