Can I buy a canopy separately? (I want to change color or if it's missing).
Well, not really, I never saw the Mothercare's Amble sun hood offered just so, in their shops, nor did I see it on eBay and similar sites that usually offer these things... The general answer will, therefore, be a no, BUT...
Try contacting their customer service. Many brands (e.g., Valco Baby), do tend to offer the canopies at least as a spare part, so they might be able to order or supply you with one of a different color.
Also, please leave us a review of the Amble to help other parents make their choice, just as we try to help you! Thank you!