Selection via Color Range
This example starts with the rough sketch on yellow legal paper, which is scanned into the computer.

Duplicate the layer to preserve the original sketch. To eliminate the yellow background, choose Select...Color Range.

After the Color Range dialogue box appears, use the eyedropper (not shown), and place it on the yellow color in the document. The preview shows the yellow areas in white. Click OK, and those areas in the document are selected.