Brush Colour Problem

I am having this problem and need help with the brush colours. I am using PS CS4 and below is the problem stated:
I select a layer colour it black and now i want a white brush stroke on it, but when i select the brush and also select colour white it does not work, The Brush works allright but the problem is colour. The brush stroke's Background colour is always that transparent background which appears before colouring the layer.
Now please tell me what is the problem here and how can i change the colour of the brushes?

Bump,

Well maybe its that its not in RGB, go to your colour panel press de little arrow that at the top right of it and go down to RGB and select it.

Dunno if its that but it may bee

Nop i have done so, but the problem still remains.
here is a preview image.

see you can see by preview it shows white because it can not show the tranparent. But if you say thats how it do work then how to change the white colour to something else? i mean what if i want that animal to be brown? thats not happening