Set BufferedImage to be a color in Java

I need to create a rectangular BufferedImage with a specified background color, draw some pattern on the background and save it to file. I don't know how to create the background.

I am using a nested loop:

BufferedImage b_img = ...
for every row
for every column
setRGB(r,g,b);

But it's very slow when the image is large.

How to set the color in a more efficient way?