Unless you're red/green colorblind.
About 10% of males can have problems with red/green colorblindness, varying with the population you are considering. Most colorblind people don't just see gray, but it is very common for them to have problems distinguishing particular hues and intensities of red from corresponding greens.
If you are designing a product that uses red and green to display important information, then make sure there is a secondary way to obtain that information that works even if you can't tell the colors apart. Some example strategies include:
- Positional information. Traffic lights are OK because the red light is always on top, so you know what color it is by its position.
- Use color only as auxiliary information. If the display is the red text "FAIL" vs. green text "OK" then colorblind folks will do just fine.
- Blinking rates. If you have a bicolor LED, then consider flashing for red and solid for green (which may be a good idea anyway, since flashing lights attract attention). Or a distinctly different blinking rate.
- Significantly different luminosity or brightness. A very dark red vs. a bright green may work out OK, but you should do some testing or dig deeper to be sure you got it right.