Reply To: Why does my Background image color changes when I place an image ?

Using Transparency forces the display to use “accurate colors” for your current Document Color space. Since not every color can be converted accurately from RGB to CMYK (InDesign's default color space), ID converts them to the nearest possible value.

You are not printing the document — which does need it to be in CMYK space –, so you can simply change the blend space to RGB. It's in the File menu, I believe.