If you make a New Layer in Artrage you will find you can outline and “block in’ each object of a still life on a separate layer while still maintaining the complete painting.
The Padlock icon
Then if you click the right icon in the layers menu (the padlock) you can lock all the transparent pixels on that layer and you will find you can paint freely within the outline of your apple or bottle! Build up its colour changes and the form of the object with light and shade.
Paint or tint or draw
I prefer to use a small area of the painting on a new layer, rather than separate objects (right). In order to define my edges clearly I can zoom in or alternately I can leave the drawn lines as part of the painting. Also to tint an imported ASKetch drawing (top right) use the transparency icon of the layers menu. It depends what you are painting as to which way you work. You can also add highlights on a layer and then merge down, try out ways of painting, delete layers etc.
Read the Artrage instructions and experiment!