You can purchase grey or brown rendering sands. I am guessing the end product would therefore be either grey or brown. Assuming that the grey render sand is only mixed with lime and water it should retain its 'greyness'.
Building Sand is often much finer than rendering sand. I realise that the builders merchants didn't have specially designed render sand, but for the best and ultimate quality you really should source rendering sand. I don't know much, but I know sand!