The most important thing to know here is that while every screen on every computer, tablet, and phone will show the same image slightly differently, when we print we go directly from your file to the paper (using top-of-the-line printers, ink, and paper, of course). We don't apply any changes or conversions of any sort in order to stay as true to the file you uploaded as possible. You should still be really happy with the finished product, and if we think you won't be, we'll contact you before printing it.
For our technical customers: the most likely cause of this is that your file is a JPG in the print-ready CMYK color space, but your web browser is rendering it in RGB. If it appears correctly when opening the same file in your favorite graphical editor, then you can be assured we'll receive it and print it in that color space. If you have any questions about this, please reach out to us and we'd be happy to help!