I understand that you are trying to color in the areas set by the line art, correct?
To do this, open any line drawing in Rebelle via File > Open. If the linework has a background you need to remove it via "Filter menu" -> "White to alpha" (if it is a white background) or "Black to alpha" (if it is a black background).
After you apply this filter you will have the lines on a transparent layer, Canvas is placed below this layer.
You can set the linework layer to "Masking Fluid Layer", add another layer which will be "Influenced layer" and place it below.
Now paint on that layer and the linework from the masking fluid layer will serve as a masking fluid.
Here are some useful videos on how to use 'masking fluid' and 'white/black' to alpha filters:
Let me know whether this helps.