Do I have to prime the wall to get the exact color of my choice ?

No, you don't. If you were putting a very very dark color (red or black, etc) over such a light color, you'd want to prime with a tinted primer; on the other hand, if you were trying to cover up red or black, yes you'd want to prime. However, the colors you're talking about are that drastic and I personally wouldn't prime. However, be warned that paint looks different in rooms because of light(ing) - even looking different at different times of the day. So, before you paint the entire room, buy one of the small sample jars and apply an area to your walls and live with it a few days, or just buy your paint and do an area and wait a few days. Paint does dry darker and it does get darker with 2 coats if you're skimpy with the paint when you put it on. This way, if you aren't content with the dried color, you can then choose to use a tinted primer or plan on 2 coats of your paint, or even buy a different shade of paint. You don't always have to use 2 coats of paint if you apply the paint properly and if you've bought a good paint. However, if the current paint is filthy or greasy you'll either need to wash the walls or use primer or Kilz. Be sure to wash your baseboards and door and window trim, as well as vacuum cob webs off the ceiling, because you don't want to paint goop into the walls. Another tip is to remove fuzz from new paint roller by cleaning it mailing or duct tape.

No. You do not have to prime. More than likely you will have to paint two coats but that is true of all paint just about. The only time you may have to prime is if you are trying to cover over a dark color or red paint with a lighter color (but I use GRAY paint in that situation instead of the normal white primer because it covers up the dark color better).

Another reason to prime first might be because you are using a glaze rather than full color. In the case of glazing it will cause a sheer effect and some color may bleed through the top color.

In your case because you are doing a darker over a lighter color, you should have no problems with it. The color won't bleed through. You will have to do two coats no matter what. Once you do that second coat of swiss coffee it will be like what is on your paint chip. I have found that paint stores will tell you that you have to prime all walls before painting....keep in mind they are trying to sell paint.

Yes. Definately prime it first. If you just paint over it, you will still see the yellow and pink and it will distort the coffee color.