sounds like you're trying to do exact matches and that does take time. Consider instead just looking at the contrast between the similar fabrics...for example, there are three shades of blue in the top of the Ganon window (for the sea). Instead of working on the exact shade of blue, look at what contrasts nicely to give the general look. Once the quilt is done I doubt that anyone will spend time comparing the original with the quilt to see if you found the exact match.
Also, to make it easier, insteading of adding leading on top of the colors, have you thought of doing raw edge applique and leaving a bit of the black background showing between the pieces? The overall look is the same, and it's much easier.
Challenges are good, and I"m sure you'll get it done and it will be beautiful.