There are no effects on the star, and changing the opacity of its stroke or fill doesnt change anything. I've also tried placing the star behind the jpg, and also moving it to a blank area of the page, but it seems like my jpg's colour get dulled by the mere presence of the star on the same page.
Please make sure you have the same document color profile selected in both Illustrator and InDesign (CMYK or RGB) so that there's no conversion happening. Also check if there's any transparency applied, which could be causing this change in color.
but when I place a vector object (the red star) over it, the jpg's colour changes and becomes duller. Why?
The vector object has some kind of transparency applied. You can set your Transparency Blend Space to RGB if you want to keep the JPEG's RGB colors.
Thank you both for your responses! Everything is the way it should be now.