Before you can apply colours to a black and white page, you’re going to need to remove the white from the page so the colours will show through under the black lines.
In Photoshop, a very quick way of doing this is to set the mode of the layer with your artwork on it to ‘Multiply’ instead of ‘Normal’. This will immediately make any white area of the artwork transparent. So, if you’re fancying some quick’n’dirty method of page prep, this’ll do.
However, it’s not perfect – it doesn’t allow you to do colour holds by colouring the lineart for example.
A far better of prepping your art is to use the channels in Photoshop. It’s a wee bit more involved, but so much more flexible.
Select all of your lineart layer, and hit ‘copy’. go to your Channels tab, and create a new channel. Paste all of your lineart into this new channel. Go back to your main layers, and delete your lineart layer. Now before you shit yourself in fear, it’s ok – your lineart is saved in the channels. Finally go to the ‘Select’ menu, and ‘Load Selection’. Pick the channel with your lineart on, and your lineart will appear as a selection on the layer. Just fill that selection with black, and lock the pixels on it. You now have lineart with no white at all, and as the layer is pixel locked, you can colour the lineart directly to really let explosions, laser fire and stuff look great.
This works fine for web – for printing you’ll need to have another layer under your lineart that will allow black to print properly, but that’s a lesson for another day.