Using the Highlighting Tool you can create a Text Red style (see below) in a new Palette. Once that you have created this style, then you could set that Palette to send all your red-lettering highlighting to a Specific note file (e.g. Red Letter NASB).

Below is a screen-shot of my red-lettering for the NASB in Genesis.

(Click on the above image to see the image in its actual size)
Edit:
i just created a FaithLife group: sclark Red Letter OT
You can go there and look in Documents and get Genesis HL nasb highlighting file and add it to your documents. This gives you an example of how your Red Letter Old Testament would look. Obviously to view this highlighting you will need the New American Standard Bible 1995 in your library. After getting my highlighting file (document) you should be able to go view Genesis in the NASB95 and see the red-lettering (you may need to ensure that in the NASB95 that you have Genesis HL nasb checked in the Visual Filters (3 circle icon)).