That seems strange. I would expect that JuiceboxBuilder-Pro resizes images and thumbnails using similar rescaling algorithms and techniques and all metadata is stripped out of both images and thumbnails during the resizing process.
Also, the main images and thumbnails are both displayed in the gallery using (dynamically-generated) standard HTML <img> tags.

Are you allowing Juicebox-Pro to resize your images or have you deselected the 'Resize Images' checkbox (in which case JuiceboxBuilder-Pro would just copy your original images to the gallery's image's folder)?

If you see the problem in only certain browsers or devices, then it sounds like it might be an ICC Color Profile problem.
It sounds like your images may have ICC Color Profiles embedded within them and that JuiceboxBuilder-Pro is not resizing them. This would result in images which retain their ICC Color Profiles and thumbnails which have the ICC Color Profiles stripped out. This might account for the differences between the two sets of images.

There are no color settings within JuiceboxBuilder-Pro (and no way to force JuiceboxBuilder-Pro to retain metadata in resized images or thumbnails) so to ensure that the images and thumbnails look the same, you might need to either:
(1) Allow JuiceboxBuilder-Pro to resize you images.
... or:
(2) Create your own set of thumbnails with your ICC Color Profile. This should result in your images and thumbnails always matching each other but perhaps not always being displayed consistently across different browsers (which may or may not support ICC Color Profiles).

It is usually recommended that images intended for the web should be saved as sRGB.
Please check to see if your images are saved as such.