Can PDTextField set a text color with an alpha value?

I am currently setting the default appearance string to set the text color like this:

String defaultAppearance = "/Helv 12 Tf 0 0 1 rg";

field.setDefaultAppearance(defaultAppearance);

I cant seem to find anywhere if there are other options for the formatting string other than rg for RGB or g for black and white.

Is there a way to set the text color to an rgba color, set the text alpha, or documentation on the format of the default appearance string I could look at?