Lighting the atmosphere can get very complex, especially when you add clouds to the mix. Personally, I don't like messing with the bluesky colour because I still like to have blue sky higher above the horizon.
An alternative approach:
Set GI render settings detail: at least 2, quality at least 3 (I usually start at 3,4 respectively)
Go to the Envirolight node and set the atmosphere tint to a reddish colour and the surface tint to a bluish colour (I use the bluesky colour). Toy around with increasing the strengths separately
Give the sunlight a slight yellow or orange colour.
The sky for a sunset is usually relatively uninteresting in real life... and TG does a relatively realistic job of that. Add some clouds.
Change the colour of the clouds so that the higher clouds are lighter than the lower clouds. The combination of red decay, GI and your sunlight colour will then provide some interesting colour variation. There are many other tweaks you can do with clouds. I tend to try an approach that will work for all sun elevations, but many people change the colours as well for extra effect.
The other tweaks I use include the atmospheric glow power (increase amount, and reduce power using values such that amount x power is roughly equal to 1), but once again, it's up to you.
