Separating hand drawn sketch from background

Usually, removing the background of an illustration is handled efficiently using only the Blend If command on an RGB file. In this case, though, some of the lines are so thin and light that additional steps are suggested in order to maintain greater image fidelity. These are the steps:

Open the file and change its mode to Lab Color. Choose the L channel and Select > All followed by Edit > Copy. Then, deselect the marching ants.

Return to the Layers panel, add a blank layer and Edit > Fill it with White. Then, Edit > Paste, which will put the image, the L channel copy, on a layer above the white layer. Turn off the Background layer.

Double-click on the top layer to bring up Layer Style and set the top slider as shown. Note that the background goes from a very light gray (84 on a scale where 100 is white) to white except in the lower right corner. Use the Brush set to White to remove the gray value.

Move to the middle layer, Select > All and tap the Delete key. It is now transparent. Then, deselect the marching ants.

Return to the top layer and Layer > Merge Down. Add a transparent layer between the existing two layers and Fill that layer with a color. I chose blue for this exeercise. I would actually prefer ivory or something similar. It's your call.  Choose the top layer and change its Blending Mode to Multiply. Last return to RGB Mode.