Poor service delivery

I purchased paint at Builders Kempton park as I only needed 5 litres not 20 litres,and was informed that only the texture would be different but colour will be same.Upon painting the colour came out to be different and I informed them, was advised to return it , I had already lost slip ,but upon arrival was told that my purchase could not be traced thorough the builders card , my issue bus why a sudden turn around and why sell products without product knowledge this is second time having problems with Builders warehouse at Kempton park ,they take customers for granted