Some PDF documents change the colors when they are used as background for another PDF. This is often the case for PDF files created with other tools where the color information is not fully compatible.
Print PDF to a new PDF with Bullzip
You can often get the colors right if you print the existing background PDF to a new PDF using the Bullzip PDF Printer. Simply open the background PDF in Adobe Reader and print it to the Bullzip PDF Printer. Then use the new PDF as the background PDF.
If this doesn’t work, you can try the same procedure with at different PDF viewer. Instead of Adobe you can use Sumatra PDF Reader, which is a free reader available for download on the Internet. This has often produced good results.
PDF Compatibility Level
You can also try to change the compatibility level of the PDF that you create. This can also be used when you create the background PDF by printing it from another reader as mentioned above.
PDF Color Model
In the PDF options dialog, there is an option to change the color model used, when creating a PDF. Try to experiment with that as well.
Hopefully, one of the strategies mentioned here will help you solve the color issue when using another PDF as a background. These methods should all preserve the vector based information intact and deliver the best quality possible.