Exposure is a mix of Apature, Shutter Speed and ISO setting, on the EOS 400 you can set these Manually or in Av or Tv modes then opt to braket the images. This is probably the best choice if the object is stationary and is something the camera can be set do automatically.
The white balance will change the colour temperature, which may give the impression that the image is underexposed.
To gain maximum control you could choose to shoot Raw. This will allow you to control the processing of the captured image rather than using the camera's pre-progammed settings. This will allow you to make changes to exposure, colour temperature, contrast, brightness etc.
One thing to note when shooting Raw is that the histogram and review image dispayed on the cameras LCD are subject to the camera's built in processing so these may appear different when opened using a Raw convertor.
I choose to set the camera to 'Faithful' and Auto White Balance, and aim to have the images appear slightly overexposed. This is because of how the camera records detail on the chip. Typically DSLRs have a dynamic range of 6 exposure zones. Say Zone 1 is the darkest and Zone 6 the lightest. Zone 6 records twice as much information as Zone 5, and Zone 5 twice as much as Zone 4 and so on. Therefore 'exposing to the right' should provide a higher level of information recorded per shot, allowing more flexibility when processing the Raw image.
Of course there is much debate on these topics and the above is a simplified summary but hopefully it provides some help
