Color exchange not working

[She]

Hello. Thanks so much for your advice about rotate colours. I've had a play with it and it is a really useful tool. My struggle with this particular picture is that the light blue colours are just right. I'm wanting to change the lilac / purple to shades of dusty blue. The rotate colours also changes the colours I want to keep and I can't find a way of keeping some blue and also getting rid of the lilac / purple.

Is there a method where I can select specific colours in the image and swap them for specific other colours? Or a way of making the image a more unified dusty blue (without purple shades)?

I appreciate your help.