Does Dyeing Your Hair With Coffee Really Hide Gray Hair?
I haven’t found any research or demonstration that this works for most people. Many sites that claim it actually works (like Death Wish Coffee) promote coffee, and even then, this site offers several exceptions.

So, is dyeing your gray hair with coffee really effective?
After reading this for (too) many hours, it seems like this method will work if your natural hair color is several shades of coffee color and you have a small amount of gray creeping in. If you’re not sure if this method will work, you’re probably wasting your time.

Coffee seems to work as a general hair dye, however, if you are looking to darken the overall shade of your hair a shade or two.

Final Words
Overall, coffee is a safe and affordable way to temporarily cover up gray hair. It is easy to use and leaves your hair healthy and soft. Dyeing your hair without harsh chemicals requires very little effort when using coffee as a natural hair dye.

However, keep in mind that the results may not be as drastic as store-bought dyes and you will need to reapply them frequently to maintain the desired color. Also, wear old clothes when applying this mixture, as it can stain easily!