factor_* is pretty easy. 1.0 is the default size (in x and y), change factor_x to 0.5 and it's half it's original size. 0.25 would be a quater of its original size. 2.0 would be double it's original size. etc.. (same for y obviously)
I have no idea what you can put in mode.
The color is default set to white in order to display the image using it's default colours. Change it to 0xaaffffff to draw a transparent image, and/or change the rgb values to tint the image to another colour, like red: 0xffff0000 The format is 0xAARRGGBB, where each pair can be anything from 00 to ff, in hexadecimal. Therefore, 0xffffffff is Alpha: FF (fully opaque), Red: FF (very red), Green: FF (very green) and Blue: FF (very blue). Add these together and you get bright white.
I'm sure someone can give you a better explanation... I'm having a cold and fever and isn't thinking straight. :P
Mode can be either :additive or :default. :additive will just add the new colors to the underlying ones, so if you draw a lot of grey additive particles over each other, they will eventually become white. Not so with :default, but it looks nice for e.g. smoke effects.