AC powered light might change colour slightly due to the mains frequency, while it's normally the brightness it is sometimes the colour temperature. It could also be the brightness change of the light, the camera applying a bluer white balance to the scene, but less of a warm light from the lighting.
When you say the white balance is set manually, what does that mean?
Canon cameras have auto white balance and then several white balance presets such as tungsten, shade, cloudy and fluorescent, plus there is custom white balance and for some cameras kelvin white balance.
Only when set to custom white balance or Kelvin will the same white balance be used across the whole frame.
When a preset like tungsten, shade, cloudy, fluorescent, daylight etc is used these have a little "wiggle room" and the camera can apply different white balance to different parts the image to make an overall pleasing image. Usually the difference is a few hundred Kelvin, but it might be enough for your images. I'm assuming these are JPG images from the camera, if they are RAW and processed in Canon DPP then you see similar, but when using say Adobe Lightroom they use a fixed colour temperature for the whole frame based on the temp and tint sliders.
Brian
EOS specialist trainer, photographer and author
-- Note: my spell checker is set for EN-GB, not EN-US --
