If you want to use your scanned drawings and color them in Photoshop, here's a fast and easy way to get rid of all the white in the background.
Open up your scan in PS. It will automatically be on a layer called "background." This layer is locked, so double click on the word "background", then you'll unlock the layer and can rename it in the dialog box.
Then add another layer and move it under your initial layer. On the original drawing layer, go to the layers palette and change the layer state to multiply. This will get rid of the white!! Then you can paint on the layer beneath it. Easy Peasy!