For an Inside Mount, you specified the exact size of the WINDOW (make no deductions!). The factory will make your blind slightly narrower.
For an Outside Mount, you specified the exact size you'd like the BLIND to be made. The factory will neither add nor subtract anything to those dimensions. You should include room for mounting the hardware.
Each computer screen is different. If you require an exact color match, you should consider ordering samples, so you can be sure you love the color you are ordering.