Showfoto tries to give you as much control on how it works as possible. There are many options that change the behavior of Showfoto. To access these settings select → from the Menu Bar. The configuration dialog is separated into eight pages. You can change between these pages by clicking on the icons on the left-hand side of the dialog.
The Interface Options section is rather self-explanatory.
The Exposure Indicators lets one choose the color to use when the under- respectively over-exposure indicators are selected. Preferentially the colors should be contrasting with what they indicate, that is, under-exposure should have a light color and over-exposure areas should show dark colors.