This is how I make my glasses for skins. I use McSkin3D which allows me to use transparency for the lenses.
For the frames, I make either of the two pictures below depending on what kind of glasses I want to use
http://imgur.com/a/sw0Zihttp://imgur.com/a/SdZi4Then, for the lenses I go down to the "A" part of the RGBA bit of the color wheel and go about halfway for transparency. I usually use either white or gray if the lenses don't need to be any specific color. With the transparent lenses added, it should look like this:
http://imgur.com/a/CNz6GYou may want to mess with how transparent the color is and what color you use to see what you like best. I hope this helped!
