Prohibited from fitting white garments

I was at Mr Price in Balfour Park the weekend before this one past and wanted to try on some items of clothing. At the fitting room, I was told that I was not allowed to try on the white and cream colored tops which were part of my selection. The assistant told me I must find the same shirt in a different color to try for the fit & I can speak to the manager if I have a problem. I didn't bother with the tops. Firstly, I don't see how finding a shirt in a different color helps me when I want to see how WHITE looks, secondly, there were no similar tops to try, thirdly, how can a consumer buy clothing if they are prevented from trying them on