1. Classic Color Meter (free Version Download For Mac
2. Classic Color Meter (free Version Download For Mac)

If you want to avoid installing anything, you can use Microsoft Paint. You can open a saved image in MS Paint, click the color picker (looks like an eye dropper), and click the part of the image you want the color for. Then go to Edit colors and you can see the RGB or Hue/Saturation/Lum.

Classic Color Meter (free Version Download For Mac

Classic Color Meter (free Version Download For Mac)

If the color you want is not already saved as an image, you can use the built-in Snipping screenshot utility, then follow the steps above. MS Paint enables zooming, so you can get down to the pixel you want.