Here's what you need to consider:
* Temporary dyes: These generally deposit color on the hair shaft, not inside it. They tend to fade with washes.
* Gray hair: Gray hair often has a coarser texture and can be more difficult to dye.
* Dark brown hair: Dark brown hair has a lot of pigment, which can make it harder for a temporary dye to show up.
Here's what might happen:
* The temporary dye may not show up on your dark brown hair at all.
* The temporary dye may show up on the gray hair, but not the dark brown. This could create an uneven color.
* The temporary dye may not last very long. It could fade quickly, especially if you wash your hair frequently.
Recommendation:
It's best to avoid using a temporary dye on both your gray and dark brown hair. If you want to try a temporary dye, it's best to use it on just the gray hair to see how it works.
Alternatively, you can consider:
* Semi-permanent dyes: These last longer than temporary dyes and may be more effective on gray hair.
* Professional coloring: A professional stylist can help you find the best hair color for your specific needs.
Before you dye your hair, it's essential to do a patch test to make sure you don't have any allergic reactions.