Usually only works for the darker color signatures.
You can try to make the a flare off of sight, but it's lighting effect still touches the focus area.
Before you softglow also: It sometimes helps to raise the contrast and lower the brightness before you apply the effect, also try to set the (I dont have GIMP open ATM, so I don't know it's exact name) try to keep the setting that spreads it out low, up it to 0 then up it up a little, while keeping sharpness and brightness to 100 (or brightness to desired level).
And I don't know if you've experimented with it yet, but there is a option on the layer's menu (usually says normal) try changing some layers to say multiply, overlay and screen. You'll discover some nice effects