My opinion on this is, it depends.
Surfactants, the cleansing agents in shampoo, will remove the dirt and oil from your hair, including your scalp. These natural oils act as a barrier protecting the hair and scalp from the color, which could irritate the scalp depending on how sensitive you are.
Generally the rule is, wash your hair 24 to 48 hours prior to a color service. But again, this depends.
If you have really oily hair that’s obviously oily after 48 hours, please stick to the 24-hour rule. I have a high tolerance for oil, but it’s hard to do an accurate highlight when there’s so much oil separation.
A very lovely client of mine notifies me ahead time if she hasn’t washed her hair and offers to pay me more for the inconvenience. This is only when it’s been like 4 days. Bless her heart.
Also, check with your stylist, because when to wash could depend of what kind of color is used. If your stylist uses an all-over demi-permanent color, this is best done on clean hair. Clean could be, washed that morning but no scalp-scrubbing. This color is less invasive and usually has no ammonia or other irritating chemicals.
The safe bet all around is to wash 24 before color.
Be kind to your stylist, acknowledge your oils!