With a half dozen sets on Glocks firing thousands of rounds, my advice on visible sights for old eyes is HiViz.
They make a combination of front and rear that has been visible and durable. The front is larger than the rear so you can tell which is which. It fits nicely in the space between the two rear dots.
Green green green is the answer. Yellow is hard to see and red is so bad it is hard to see except in bright light. Green sticks out very brightly.
The front installs with a screw and sight. The rear is a dovetail with a lock screw. Being a belt and suspenders kind of guy, I Epoxy Steel the bottom of the front sight to the slide. When the rear is positioned for windage, I flood the base and Glock dovetail with Super Glue. Neither front nor rear has ever moved on my sets.
The HiViz phone number is 1-800-589-4315
The HiViz numbers to ask for are: Front=GL2009
Rear=GL1005 for 9mm/40 slides
The lady on the phone does a very good job. If you have technical questions, Rick Moore knows everything.