I believe it is a bug because it is an unintended side effect of the pre-processor which means you can’t use the color method with an off-screen buffer.
In the example I posted I change the color mode for the buffer and then try to calculate a color value using that mode. I can’t simply use color(…) because it would use the main display color mode.
In Processing the data type color is an alias for int so v.color( gets translated to v.int(, hence the error message.