Henna hair dye can take days for the colour to fully develop. A process called oxidation.
So we always recommend doing a small 'strand test' on your hair (usually a strand behind the ear somewhere discreet or you could even remove some hair from one of your hair brushes to test with).
The henna oxidation period can go on for as long as three days. So your hair will change colour slightly for three days after you have washed the henna off.
This will be most noticeable if you are going from a very light blonde shade to a copper or auburn tone.
So remember... doing a strand test somewhere discreet and leaving it for 3 days will show you the genuine colour you are going to achieve.