Yes, but you need to find out how they came to be brown and paint them accordingly. Sometimes it's aluminium foil with a coloured pattern on stick to the pvc, sometimes it's brown paint on white pvc, sometimes it's brown pvc..
There are a number of products that claim to be suitable for painting uPVC. I haven't used any but would be inclined to think that the nasty solvent based ones are more likely to be able to etch the surface and be more durable.
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