Pink (?) Phalaenopsis

About ten years ago my brother bought me a pink moth orchid - it was a very delicate pink -

it has flowered regularly and abundantly all these years, but I have recently noticed that the colour is changing...it is getting rather dark leaning towards lilac. Is this normal for this orchid? Am I doing something wrong? It sits on my kitchen windowsill and it seems very happy there together with other phalaenopsis.

Can anyone give me some ideas as to why it is changing colour?

Many thanks