This could be due simply to these maps containing both very small and very large values, maybe in non-brain regions or places where the fitting is poor. Depending on how FSLeyes figures out the intensity range to display, this might look like what you’re describing. If that’s the case, I’d expect to see more or less the same thing in mrview – what does that look like? In both cases, it would be a simple matter of adjusting the windowing (on mrview: right-click & drag, or use the View tool to set the min/max of the intensity range; not sure for FSLeyes).
If that’s not the issue, maybe you can post a screenshot to give us a clearer idea of what the problem is?