Learn about Layering
In any pattern that has more than one colour, you have to decide which colour to put on in which order. What you choose will make a difference to the end result, but it's much harder to change your mind once you've started wrapping the thread about the nails.
Take this ship for example; the elements of the image are the sails, the flag, the mast, the boat and the sea. If I start at the bottom and add the sea first, the brown wool on top of the blue wool won't look as good as the blue waves over the boat.
I could decide to skip the mast and just have two brightly coloured sails, but I might also choose to have the mast in brown behind (or in front) of the sails instead.
I suppose perspective might also be a good word to describe what you are doing with layering.
The final level of layering is the outline and for the most part it hides the join between the two colours, but it's worth thinking about which way around the nails you will wrap it.
Head over to my page on outlining to see what I mean.