With the help of Wikipedia, and some knowledge of my own, here’s a bit more of a concept description for both:

Radisson Blu is a full service brand, mainly located in major cities, key airport gateways and leisure destinations. It is described as an "upper upscale" hotel brand.

Radisson Red is more of a select service brand, targeted to guests with an "ageless millennial mindset", incorporating more art, music and fashion into its services and offerings with a focus on communal spaces such as the bars and lobbies, digital customer service, and bold color interiors. It has its own branded restaurant, called OUIBAR, and is marketed more at millennials. The brand's emphasis is on modern design, technology, and quick food service.