When you're cooking a steak or burgers, or even chicken breasts for that matter, you don't see what color the inside is until after you serve it and your guest starts eating.
So you can't go by color when you're cooking, unless you want to serve a sliced-up steak. Being color-blind is no hindrance to cooking a great steak.
When cooking steak or chicken on the grill I always, ALWAYS use an instant-read thermometer and take the internal temperature of the food I'm cooking. For Rare, pull it off when the middle hits 125. Medium rare, 130. Medium, 135. I don't cook steak any more than Medium.
Once you hit your target temperature, remove and let the steaks rest for 10 minutes. As you let the food rest, there will be some carry over cooking to hit the final internal temp of 130 for rare, 135 for medium rare, and 140 for medium.
Chicken breasts come off the grill at 165. Baked potatoes, 195.
There are fancy Bluetooth thermometers that have multiple zone reading, and can cost upwards of $200. But this is what I use, and it's under $20 on Amazon:
But doesn’t the temp keep rising after you pull it off the heat (and so shouldn’t you take it off below the target temp)?
The transmitter (black) has a magnetic back that allows for sticking to your BBQ. With two AAA batteries, it runs for a long time. The receiver can be a long distance from the transmitter.