Change RGB colors with opacity to Hex (whilst keeping the same colour and tonal values)

Hi

I been give a very length and complex style guide with 15 colours and each one of those colours also has a version at 75%,  50% and 25% opacity.

I'm doing a mock up of a new site and want to play around with all the different colour/opacity levels.

My question is, is there a quick way in Adobe Illustrator to get the equivalent #hex code for a colour with reduced opacity.  I know I can go into the colour sample itself, get the rgb color, look at the opacity percentage and then work out an rbga, but for instance if i take a screen shot of a color at a certain opacity and then use the eye dropper tool on the image to get the colour it gives me the equivalent colour without any opacity value, which is what I would like.

Any ideas would be awesome. 

Paul.