Ok, this is a good start.
First off, CataBlog does not officially support SeaMonkey, but it seems that it would be very similar to Firefox. Would you mind trying other browsers to see if that is the problem.
Also, when does the message appear? After you selected a color with the mouse, what did you do? did you save your options? Did you look at other option tabs? When does the message appear, after saving? What admin pages does the error appear on, just the CataBlog Options page?
If you do not check keep aspect ratio, than you could not see the color in the preview. You were only changing the color behind the preview image, which is scaled to fill the entire preview area. Is this what you wanted? why? Please, check keep aspect ratio for a visual explanation and to see the selected color in the preview.
I did not write the color picker widget, that is called farbtastic, which is part of WordPress, Drupal, Twitter and others. I don't think the color picker itself has this bug.
So, I haven't seen this message before and after hearing about it from you and trying to recreate the situation that causes the bug, I still haven't seen it. I need to be able to recreate the bug to fix it. Has anyone else seen or experienced the error message reported above? If so please help by explaining exactly what you did to reproduce the error.