For goodness sake...
I can't believe the number of people commenting who are bleating that ''rules are rules'' or that it is ''unprofessional''. Are you really that much of a sheep?
What on earth should an employer get away with dictating what you can do with your hair?
As long as you are clean, tidy and wear decent clothes what exactly is the issue with having a slightly brighter shade of red?
Hardly a case of someone wearing a punk mohican or turning up with hair a radioactive shade of green...
I would stick to your guns OP if they try to cause you problems over this. L'Oreal is a well known and mainstream company and you can argue that these dyes are widely available in most stores and are hardly a niche product.
If you use hair dye the effect achieve is by definition ''unnatural' because it is not real hair pigment'. I have ash blonde hair and when I colour it every month it usually goes a bit silver or purple before I give it a first shampoo when it settles into blonde. That would be ''unnatural'' too I assume....
People who write these silly ''rules'' should get a life.
Or OP just play them at their own game and argue that you follow the Wiccan religion and that dying your hair red is part of your practice :) that should shut them up as they can't discriminate on religious grounds :)
This ''rule'' is also more likely to affect women rather than male employees so you could argue that this is discriminatory practice that disadvantages women and is a clear attempt to control how they choose to present themselves.