You can utilize rgba colors for that. But to note, transparency colors on text only most likely noticeable on "bolded" text, either it is usually combined with text-weight:bold; or you are using bolded font-family like 'arial black' or 'impact', also text-shadow will affect the color greatly and most of the times you will likely not used it together with that.
IE8 and older do not support it.
Example done code, see the vertical text on left profile part:
http://www.gaiaonline.com/profiles/reinasachiko/24287935/
example code used in there
#id_details h2 {
font-family: Impact, sans serif;
font-weight:normal; /*change to bold for non-bolded fonts*/
font-size:52px;
color:rgba(255,255,255,0.5);
}