Pattern Coloring in xamarin

I am developing xamarin forms app which has a vehicle Image as a pattern and that can be colored by a user. I searched and find SKiasharp to handle painting part but my question is how should I define the pattern in which when the user is painting, it shouldn't come out the black lines in the image? should I define whole image point by point as a path in skiasharp? Is there any other solution rather than this for recognizing the pattern from background image?