Draw line opacity in Xcode for Titanium

I'm new with xCode and I have a problem with it. I'm making an application for my Ipad. I want to highlight text, but I can't set the opacity of my line. I searched the internet, but i didn't find any information on it.Now Im using this drawImage.alpha = lineOpacity; but this is not good, everything I draw before gets that opacity.I just want the alpha to be done on the new draw. im using this cod

[illmai]

CGColorRetain([UIColor colorWithRed:1.0f green:1.0f blue:0.0f alpha:0.5f].CGColor);