If your goal is truly to get the piece to coffee color, suntaning and or no amount of shellac will get you there. Although they chose cherry wood, they don't really want the color of real cherry. You will have to use a dye, or stain and or glazes. You can get it close with these and then use a dark or tinted shellac to creep up on the final color if needed. The customer should give you a sample of the color they want so you can best duplicate it on the wood you have used. It sounds like they may want your piece to match something they already have. If so, a piece of that should be in front of you when you are working on your piece. If you are trying to read their mind on the color, it's going to be difficult for you and they might not be happy with what you thought they wanted. Good luck.