I've made various compositions, so at the end I could join them all in one. But when it renders the color seems to become dull.. and some solids become semi translucent... is there anything I could do to fix that?
Could you be a bit more specific, please. There are many places where this could go wrong.
If it's not a problem with the compression, you might not have 100% opacity on the solids or the composition nested - do you have a background layer? Check the alpha channel on all the compositions.