On AIX systems, aixterm terminals have an inner border between the window border and the text. The color of the inner border is always the same as the current background color. When you are using Animator, the background color can change depending on the current function, such as querying a data item; in this case, the color of the terminal border also changes. If you find this distracting, and want to prevent it happening, you are advised to remove the inner border. You can do this by setting the border's size to zero in your .Xdefaults file:
aixterm*internalBorder:0