Need help resizing images in bulks without losing quality

InfranView's batch tool is simple. I select however many megapixels I want the new images to be in and it does the rest. But the resulting images don't have the same vibrant colors. I don't see what I could be doing wrong. Maybe this is just how InfranView is. PhotoFiltre's batch tool is too slow. Is there an alternative that works perfectly? I only want the resolutions to be lower, nothing else. I don't wanna compress them.

Edit: FastStone Image Resizer has the same problem. The colors become less vibrant.

Maybe try XnView? I haven't messed around with it's batch editing features but it seems very robust elsewhere.