How to change a white shade to a different white shade?

Question

I have a question concerning the different shades of white. This is a picture of one of my sketches drawn on an A4 paper scanned to my Macbook. I wanted to resize the image so that it fits on a larger horizontal frame, so I pasted the original scan onto a white background that I had created in photoshop. To my surprise, when I uploaded it to my website and look at it on a window pc, it clearly shows two different shades of white, with the original A4 area looking grey/blueish. So I opened it in photoshop again and sampled the color and get the result below:

The white shade sampled from the circle 1 shows to be from the red section, while the white shade sampled from the circle 2 shows to be from the blue section of the color spectrum.

And the third screenshot is when I change the layer to multiply, and apply a solid color layer underneath, then it clearly shows that the layer 0 has two different shades of white on it.

My question is, can I replace the color of the "blue" white area with the white shade from the red "white" area?

Thank you in advance.

Answer 1

First of all you should make your image greyscale (Image → Mode → Greyscale), that will get rid of your tinting issues. If you need to do this is an RGB file you can just desaturate the specific layer (Image → Adjustments → Desaturate).

For the white itself you can use curves to set the white point of your image. Just add a curves adjustment layer (Layer → New Adjustment Layer → Curves... or just add it from the Adjustments panel), then use the white point eyedropper (the bottom one) and click on an area of the image that should be white (you can do the same for the blacks too if you want.

You can of course manually adjust your curves (drag the two pins at the bottom to set the black and white input levels).

Answer 2

You have a white balance error. That can be born in your scanner or it can be a mismatch in color management when transferring your scanned file from one software to another. Possibly Photoshop thinks something nonexistent of your scanning system. But the error can be fixed in your image.

Type 1 white area is under your control. Fill it with type 2 white. Then adjust the color and the levels as you want.

An example:

Type 1 area is filled by painting. I picked the color at first. I also made a selection to save the drawing from my big brush. It's the dashed line in my screenshot:

The image is desaturated (Image > Adjustments > Desaturate) to fade all color. You can as well goto Image > Adjustments > Hue&Saturation to change the blue to another color.

Next I lifted the background grey to a little more white. This phase is the place to add contrast heavily, if needed. Or skip it. I only lifted the background grey. The tool = Image > Adjustments > Curves

Finally an example of heavier contrast boost:

Most of the background is pure white and the drawing is generally more black. That can be useful if you add something colourful.

Answer 3

Use the adjustment curve and move the arrows to the beginning and end of the white curve on the graph