Lighter chocolate cake
What you are asking for is physically impossible. 

There is a reason why the color system when working with physical dyes (as opposed to colored light) is called additive. You cannot take out a color which is already there. When we are talking about a cake, cocoa powder or pure chocolate can be considered a pigment, and Guiness a pre-dyed liquid. 

The first of three methods would be to add your cocoa and guiness as they are, but try to mix them with other stuff so You can use them to make new, darker colors by mixing in other pigments, or lighter colors by diluting them. You can certainly mix white cake batter with food coloring of other colors (yellow) and the right amount of chocolate to achieve a caramel color, but this will be very far from the proportion which gives you a chocolate cake which tastes of chocolate and has the right texture. 

The second method would be to try to discolor the cocoa and guiness. But the problem is that you cannot change the color of a pigment without changing it chemically. Assuming that somebody can come up with a process which can bleach cocoa or chocolate, it will certainly change the taste a lot, if it is food safe at all. Probably not doable in a home kitchen either. 

The third thing would be to replace the chocolate entirely. But there is no substitute which tastes like chocolate. I have never heard of the "chocolate extract" which Catija refers to, but even in cases where a flavor is dominated by a single note which is easily reproduced chemically (vanilla, banana, etc.), the stuff which makes up that single note does not taste like the real deal. And if something had exactly the same composition as cocoa, it would not only taste like cocoa, it would be cocoa, with its normal color. And that part concerns the flavor only. Even cocoa powder changes the texture of a cake, while real chocolate changes it much, much more. If you could make chocolate-perfumed cake, it would still not have the mouthfeel of chocolate cake. 

Beside all the reasons why you cannot get chocolate flavor paired with a strange color, there is something else: Human perception is not limited to one sense. People rely a lot on vision when eating, and the experience of eating a strange-colored chocolate cake will not be the same "taste" experience as eating correctly colored chocolate cake. There are some fun studies with gummy bears and red/white wines which got lots of press about that, and some marketing gimmicks like the recent Mystery Mueller Milch in Germany, but this is not the place to elaborate. If you want your guests to enjoy a chocolate cake, you have to make a real chocolate cake, with the correct flavor, texture and color. 



For completeness, you have the following options instead of making an amber chocolate cake (some already suggested in comments): 


make a non-chocolate cake and use food coloring to make it amber
make a standard-color chocolate cake and decorate it with amber icing (again, use food coloring)
make a standard-color chocolate cake with any color icing, or without icing, and have some color contrast at the wedding :)