How do you plan for special colours and graphics during imposition?
Do you mean when the book/booklet is large enough to print in signatures and when one of the signatures might not have the spot color available for the press run in which it’s printed?
Do you understand the relationship between signatures and imposition and what those two terms mean? (Just asking so we can better answer your question)
I think you have the makings of a good question, but it would help if you supplied a little more detail.
In general, though, both signatures and imposition are determined by the printer and based on the dimensions of your spreads, the press being used and the paper stock.
So, for example, if you knew you were using a spot color on pages 4–12 and also wanted to know if that same spot color would be available for page, say, 83–102, you’d need to provide the job specifications to the printer so that he or she could figure out the imposition and tell you which pages belonged to the signatures where the color was available. Honestly, though, most multi-signature jobs print on the same press anyway, so that spot color is likely available throughout, but in the end, the only definitive answer will come from your printer.
You need to know which pages are going to be printed together on the same plate so you can make sure they have the same number of colours. To minimise costs it makes sense to try to avoid having just one page on a plate which uses more colours.
Make a folded mockup of your document with blank sheets of paper. Then number the pages. You must fold it in the same way that the print finisher will fold it (ask them). This gets complicated if there are more than 2 spreads on a plate. The different number of ways of folding multiply rapidly.
When you unfold your paper you will be able to see which pages will be on the same plate and you can lay out your document accordingly.
If by “imposed” you mean a booklet - Personally, I thought a multi-up layout, or a step-and-repeat, but I suppose my answer would apply to either.
If you’re incorporating a Spot color/Pantone into your artwork, and you expect said color to match in output to your swatch book, you had best inform your printer as you request your quote. Large offset presses can load a 5th station with your required Pantone or PMS equivalent, 4 color presses and digital will have to rely on their color management tools and/or print controller to attempt to match the Pantone, assuming it’s in gamut. Either one of these processes will incur additional costs, and could be a fair increase from having the job simply ran as 4 color.
It may be in your best interest to pay a little extra, and have the printer make you a physical proof of the piece. This way you can judge the colors and graphics on the actual substrate.