As you see in the title, that's the problem. I used to have it like a little shadow (The object with some alpha) And, because of the white background i can barely see the outline
here's a pic:

Thanks on before hand

First of all, which onion skin are you using? If you're using the outline one, then it will show up as what you have there.
Second, the coloured box next to the objects layer will affect the colour of the outline. If the box is yellow, the outline will be yellow. I think you get the point.

click the color next to the layer in the timeline and change it, boom, done.