The two most common causes of this issue are:
1. The file is set up as a CMYK file
2. The file still has layers and is not flattened (an issue with TIF files mainly).
The reason is that web browsers can only display images in sRGB, so when a CMYK file is uploaded, the web browser tries to display it as sRGB and ends up distorting the colours.
There is no cause for concern if you see this, the file we will receive from you will be identical to the one uploaded. It will stay as CMYK and the colours will be as per the original file and not distorted. Only the web preview will be affected by this issue.
However the issue with separate/multiple layers is one we highly recommend resolving before uploading, please ensure the layers are completely flattened in the file before saving it and uploading it.