You should try layers. They are really helpful. Here is how to use them:
When you open up the oekaki art program, you should see a layer box.
Click the + button on the layer box. You will receive another layer. Click the tab on the layer box and scroll down to the choices 'darken' or 'mutiply.' Use either one of them to color of the black outline of your drawing, but the color won't 'go over' the black line. UN-like Paint, which doesn't have layers.
(I also suggest writing your username on your artwork, so peopel know it is yours. That is called a signature.)Without layers:

With Layers:

01101001011101000010000001110111011000010111001100100000011101000110010101110010011100100110100101100110011110010110100101101110011001110010000001110100011010000110010101110010011001010010000001110111011000010111001100100000011011100110111101110100011010000110100101101110011001110010000001101100011010010110101101100101001000000110100101110100011100110010000001101000011011110111001001110010011010010110001001101100011001010010000001100110011000010110001101100101