Welcome to the Affinity forums @Budi!
The order for artboards in the exported PDF starts from the bottom. You can sort the artboards in the Layers panel if you need a different order.
Regarding your colour problem: The small images on page 2 are RGB. You can convert them manually or check "Bildfarbräume umwandeln". Safer it would (personal opinion) be, if you convert the images manually to have more control over the colours.
