I drew an animation with the fluid brush tool but I can't fill it with colour, any advice?

I would like to fill the character with colours as I intend on exporting the animation with a transparent background. However it in it's current state the animation would just be a load of black lines. So I'm trying to fill in the character with colour but with no success. I can change the colour of the lines but not add any fill between the lines. I tried the paint bucket tool but that isn't working either.

I've been watching guides ofc and turtorials but I don't remember any of them covering this. I thought perhaps I need to convert the lines into an object but that doesn't appear to have changed anything. I'm assuming I've done something wrong as I've drawn my animation but I'm not entirely sure what. Any help would be greatly appreciated