Often we see red colour when sun sets or rises. It is more rare, that sun turns red even some hours before the sunset. In this picture it was like this.
Mostly red colour is because of the Earth athmosphere, where particles like to scatter blue light more than they do red light. So blue light gets scattered and red gets through.
Picture is taken in Tallinn in the summer time. I also like the clouds in the upper part of the photo. Probably different clouds where also the reason for the colour.