White background for blue colored pencil

Or use background removal techniques to strip the entire background -- denoted by a checkerboard pattern.

This solves related problems if the other tips don't work, but probably requires tweaking...

[PiX] Easily UNCRUMPLE Scanned Documents in Photoshop! - YouTube

A screenshot of your workspace and image would be helpful.   Are you saying that the background does not appear to be perfectly white on your monitor?  If that is the case, then use the Colour Sampler tool to measure the actual colour.  The numbers never lie, and could indicate that the background is already fully white, but not appearing so on your screen.

In the example below, sample points #2 and #8 are slightly off white, but the rest are all fully white.

Could you post a sample picture?

There are a lot of ways, most likely even something a way you can automate.

In this case showing a sample picture will give you a more accurate answer from the community.