Just like @Scott has simply put it, the Live Paint Bucket tool will help you pull it off. Let me try to expound it for you.
1. I tried to recreate your Vector, composed of two compound parts, the Red petals(Rose) and Green leaves and stalk(Stalk).
2. Select the Rose path, and then choose the Live Paint Bucket from the Tools panel or shortcut key K. Make sure you select a contrasting Fill. Then Hover over the Insides of the Rose and click away. Note that the 'Rose' compound path is converted into a 'Live Paint' Group in the layers panel.
3. Go to Menu (Object > Expand) to expand the Live Paint group. Make sure the Object, Fill and Stroke options are all selected.
4. Use the Direct Selection Tool A to select one painted path inside the group, go to Menu (Select > Same > Fill Color), then group these paths Ctr + G. Do the same for the outside colored paths.
4. Change the color of the inside group to match the outside (Eyedropper Tool I) and change its opacity to 50%.
5. Repeat steps 2-4 on the Stalk compound element.