how can I Convert a grey sketch image to color sketch?

Question

I have a image converted into sketch. I am trying to convert the black and white sketch into a red and white image. Is this is able with opencv? The image I have: The original image and the sketched image in black color is this.

As you can see, the image is sketched in black. I am trying to sketch it in red or green or any other color rather than black. I looked all over the internet but could not find a way to achieve this. I know this question is very bad but please do bear with me and I am really grateful to any who can at least give me a hint to start my work. Thank you very much.

Answer 1

Quick guide rather than fully tested answer.

Load the image as greyscale:

import cv2
import numpy as np

grey = cv2.imread(YOURIMAGE, cv2.IMREAD_GRAYSCALE)

Now make empty channel of same size to use for Green and Red channels

empty = np.zeros_like(grey)

Stack the grey image as Blue and two blank channels as Green and Red

colourImage = np.dstack((grey, empty, empty))

Change the order of the 3 innermost parameters in that last line to get different colour effects. Or add 64, or 128 or 255 to either of the empty images for other effects.

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So, if you stack per my answer above, you'll get:

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If you invert the Blue channel and stack like this:

colourImage = np.dstack((~grey, empty, empty))

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If you save the Blue channel and stack with 50% Green and 100% Red like this:

res = np.dstack((grey,empty+128,empty+255))