start with copy and paste your lineart onto a new layer. If
you have any other layers make them invisible. Pull up the channels
on the 'load channel as selection button', then hit the 'delete'
button. This will remove the white areas.
notice that the lineart looks very light in colour after deleting
the white. In order to make the lines black again, first hit the
'lock transparent pixels'. Basically this means that you can only
paint over areas that are opaque. As your lines are the only opaque
areas, this means you can change their colours without affecting
the rest of the picture.
a big, thick, black brush and brush over all the area lines
want to experiment with using different colours to get the idea
of how the 'lock' opacity function works.