bgColor property
Background color of the balloon (optional). Color and opacity (alpha) values are expressed in hexadecimal notation. The range of values for any one color is 0 to 255 (00 to ff). The order of expression is aabbggrr, where aa=alpha (00 to ff); bb=blue (00 to ff); gg=green (00 to ff); rr=red (00 to ff). For alpha, 00 is fully transparent and ff is fully opaque. For example, if you want to apply a blue color with 50 percent opacity to an overlay, you would specify the following: , where alpha=0x7f, blue=0xff, green=0x00, and red=0x00. The default is opaque white (ffffffff).
Implementation
int bgColor = 0xffffffff;