Stripe (upper left) has chosen purple for their logo to show forward-thinking product development. In fact, they like to call it blurple as it has a significant amount of blue hues in it, therefore targeting double the emotions of a primary colour.
The Bold One (Black)
Black represents sophistication, elegance and bold mystery. It is necessary for every UI colour palette, however, its versatility makes it a great brand colour as well. It can be used hand in hand with white.
Looking at Revolut, the black logo brings a great contrast to their light web theme, while the white logo sits perfectly on their gradient debit cards and the dark mobile theme.