<p>Using 3D printing I'd like to create a text oversized keycap that uses several strong LEDs to make the text light as red or green, depending on status.  It's quite similar to what are called annunciators on aircraft:</p>
<p><a href="https://i.stack.imgur.com/6B1lY.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/6B1lY.jpg" alt="Annunciator Panel example without the panels being turned into MX switch keycaps" /></a></p>
<p><a href="https://i.stack.imgur.com/Jce1q.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/Jce1q.jpg" alt="Annunciator removed from a panel where it is separated" /></a></p>
<p>I'd like to create an MX keycap with this ability and size with the backlighting changing from green to red to indicate something is on or off. I understand including the MX connector in the center, but how would I print a black panel and leave the text transparent, or at least transluscent?</p>

There are a few possible ways you could go about this:
If you have a multi-material printer, you may be able to print with both a transparent or translucent material for the internal structure of the print and an opaque material for the external parts of the print. PET(G) may be suitable for this purpose but is harder to print compared to standard PLA.
You can print the entire print in a translucent material, mask off the lettering at the front and spray paint the rest of the print.
You can create a multi-part print, where one outer 'shell' is printed in opaque filament, and another piece in translucent material, which will mate together after printing.
You can deboss the lettering of your print, and with the letters face down to the bed, print the first layers in an opaque material, and then swap filaments to a translucent material for the rest of the print, spray painting the upper parts of the print to match a uniform color, while leaving the bottom untouched.
There are several advantages and disadvantages to each method:
Requires that you have a (typically) expensive upgrade installed onto your machine, but would be the most straightforward way to go about this.
Requires precise masking of your letters to make them come out looking nice, but will give you sharper, more defined edges on the lettering.
Will require you to design for tolerances for the two parts, or add glue in the post-processing steps, But could be produced en masse with two printers, one for each material.
Can be a bit fiddly, and will possibly result in an undesired look, with the lettering being debossed into the part, But would be less complex than the other options.
Pick whichever method you wish, or try all of them to see which gives you the best results.