"Color" would be if you had something white coming out with tinge of a color.
I assume that you mean that the black came out as gray instead of black, much lighter than you expected. To correct for that, you need to lower the exposure, that is dial "exposure compensation" below 0.
This effect is because how "auto exposure" function works. It tries to get even exposure on whole image, it has no way of telling what you intended to be pitch black (as in your case) or 100% white (eg when photographing a winter landscape). If you want the photo to come out predominantly dark or light, then you need to tell that to the camera by using exposure compensation.
Another option would be to use center-weighted or spot metering, when you tell the camera to look at the center and ignore the rest, but I don't know if it's included in the basic camera app. You might need some advanced photo app to do this. You can also move so close that the coin would take entire screen - then the amount of black left around would be too insignificant to lower the amount of light camera sees, but then you may run into troubles with close focus.