The relevant requirement in the NVQ standards is that you follow the dress code of the assessment centre. The centre must be a working salon with paying clients (you should not be assessed on friends) and you should wear what fits in with the salon. If this is a specific uniform (e.g. whites) then this is what you should wear; if it is casual with no uniform then you should be clean and smart with 'appropriate' attire i.e have a professional appearance.
Your assessment centre should have provided you with all the requirements and other info such as the H&S policy etc that should make what you need to wear quite clear.