This is an example of an approach I would rarely adopt.
It should be apparent to everybody that the colour at the top of the image has been added. This kind of effect can be achieved with something such as a Sunset filter from Lee Filters, but in this case I used Adobe Photoshop to creatively control the highlight end of the spectrum.
Why do this? Mainly, the image simply did not work as a 'straight' picture – there was too much mist in the distant, lower parts of the valley and this simply rendered the top of the picture to sun-on-mist glare – where all closure leaked out of the image. But the gorgeous, rich spring greens of the landscape – like a felt coat – seemed too good to miss.
It therefore became what started as an idle experiment – what could make the scene work? – until I found myself pushing the orange-pink tones to somehow 'match' the greens. My impression is that they click into place here – I like the image very much because of the way the colour opposition 'works', the orange and green seem to be in a perfect oppositional balance. And so the image works in a way that the original emphatically did not.
Whether you find this sort of manipulation acceptable is largely a creative choice. (It is something I will do very rarely, not least because nature on her own usually provides something much better.) Here, it is included because it was a creative act that led, at least speaking subjectively, to a very satisfying result. But I would make no pretence that this was the scene as originally seen. If you are going to do this kind of thing, you should be clear to your viewer that it is a creative choice.