If the ambient color of the sky is purple, than your objects will always tend to turn purple. Instead of changing only the textures of the wales you could also try to change the color of your light on the wales to reduce this purple-effect. If you want the right colour to filter out the purple, than go to your favourite paint-app (like paintshop pro or photoshop) and make an image with the colour you would like light on the wale to have and substract the purple of the sky. This should give you an indication of wich color the light should be. (This can be very tricky and doesn't always work but it's worth a try.)

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