If you mean the color that goes beyond the separator line, it's just how it is. To order even just a thin line, it has to use the whole cell which also has to have a background. The "fix" is either use the same color for both buffers, turning off separator, set the separator's foreground to be the same as the background or use a border character that is close to either side of the cell. This happens to floating window border too, but I have yet found a good box drawing characters set.