In the mean time I actually managed to sort it out, but as my problem isnt on the internet I thought i'd share for others!!
If you type in the formula in the material settings and then just press ok afterwards, " appear around the path. This makes the formula not work hence the grey box because it can register the texture change, but can't find the path! After writing in the formula you must press apply and then ok to avoid the quotation marks being added.
Hope this helps others out!