Okay, so I must clear the air about an age ol' misconception that "dirty hair" is "better hair". Not when color is concerned - Or your wedding day, for that matter. Arriving to your color appointment with a weeks worth of build up is probably NOT as good of an idea as someone had once led you to believe.
Here's a few reasons why I recommend coming to your appointment with reasonably clean hair. Grease, dirt, or buildup acts as a barrier. Period. So when your stylist applies your color to your strands it will need to break through the "blockage" in order to alter the state of your current color - natural or not
It's especially important if you are getting a "gloss", "toner" or "demi-permanent" application. Those specific types of colors are designed without harsh levels of ammonia - which aides in opening the cuticle to deposit the artificial pigment.
The Result: If I apply a product/color that lacks the ability to open the hair cuticle and I am working with hair that has a thick layer of oil on it, it can prevent the product from reaching the "cortex" resulting in uneven or non-existent tones. Same with lifting (bleaching) the hair... I'm certain you would not enjoy leaving your salon with yellow strands of hair because your color was unable to process as quickly and you are now short on time and need to leave. Oil can increase your processing time as well. Again, think barrier.
Now, you MAY have instructions from your stylist to not wash, especially if you are doing an allover bleach... The oil can then act as a protectant for sensitive skin; or the stylist may not want your base to lift as quickly. However, it's best to check first.
If you are having your color foiled the actual size of your highlight may be deceptive. Oil holds the strands together and compresses it... Your stylist may think that they are taking "fine" pieces, but it could turn out heavier than expected! Knowing this, I personally have made highlights too "thin" for fear of getting too heavy of a look. So let's avoid this problem by taking a moment to wash your hair. Pretty please?
There is, however; such a thing as "too clean" of hair. It can be springy and downright resistant to clips, weaving combs, foils, etc... Excessive shampooing can strip your natural oils causing an imbalance, which can lead to even more oil production or worse, over-drying. Not to mention over-washing can cause your color to prematurely fade; so don't do that.
With all that said, there's no need to natter on about the scientific, romantic and/or social perks of freshly washed hair - it's simply polite.