Which colored layer is the orignal brass and what is the other colored layer?
Copper is "red", adding zinc to it makes it brass and the more zinc the more yellow/gold.  However , there is always an exception. Yellow brass is usually 70 Cu; 30 Zn, but a cheaper yellow brass is ( muntz metal ) 60 Cu ;40 Zn and is a little reddish compared to 70;30. For these two alloys the color trend reverses. So it looks like the knobs are 70;30 and the plates are 60;40 ; both are brass. 

These consumer items are very unlikely to be bronze ( Cu + tin ) which is more red than brass. 

Well over 100 years ago some consumer items were bronze instead of brass. Plumbing materials are a little different as the better properties of bronze make bronze a little more likely to be used for plumbing. 

Very long ago I worked at a secondary copper smelter; they melted copper alloys into standard composition ingots. It was important to separate copper scrap into about 10 categories. Experienced people could sort tons of these alloys by color and form,  not counting the pure copper wire and tubing/pipe.