The question tag ‘isn’t it’ needs to be replaced with ‘can’t we’. This is because the sentence and the question tag must be in the same tense and the same pronoun is to be used in the question tag. Thus, the correct formation would be, ‘we can go out whenever we choose to, can’t we?’
A clear cloudless day-time sky is blue because molecules in the air dissipate blue light from the sun more than they dissipate red light. When we look towards the sun at sunset, we see red and orange hues on the grounds because the blue light has been scattered out and away from the line of sight.