Celebrity nail manicurist Michelle Humphrey, who has worked with the likes of Rita Ora and Dua Lipa, says there’s one particular mistake people make when applying nail varnish that can cause nasty discolourationCelebrity nail manicurist Michelle Humphrey, who has worked with the likes of Rita Ora and Dua Lipa, says there’s one particular mistake people make when applying nail varnish that can cause nasty discolouration