In the bottle these two polishes look quite similar, but I would describe Violetta as being more purple toned where as the Indigo is, well, more indigo!
Both polishes are of good quality and the brushes are both nice although I prefer the no.7 as it has a curve to it which makes application a touch easier. For me, I would always opt for Barry M however with a £5 No.7 voucher the No.7 works out cheaper.
Barry M - Indigo (312) - 10ml - £2.99
No. 7 - Violetta - 10ml - £7