[Python] How to use OpenCV (Python) to change specific colors without loss of quality?

I have this floor plan (link removed)

And I want to change its colors so I can use OCR to read the room numbers. To do this, I want to do the following: change all red to white, and every other color to black, so all that is left is the room numbers. I wanted to try thresholding, but I saw on the documentation that it should only be done on grayscale images, so I first ran the following code to grayscale it:

import cv2
import os
from ConvertSVG import svg_2_png

# Convert the SVG to a PNG
output_path = os.path.join('converted svgs', 'Level 3.png')

svg_2_png(os.path.join('svg', 'Level 3.svg'), output_path)

img = cv2.imread(output_path)
gray_image = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

cv2.imshow("Grayscale", gray_image)

cv2.waitKey(0)


And I got this output (link removed)

As you can see, the grayscale did work, but the room numbers got blockier and harder for OCR to read.

How could I use OpenCV-python to change all red to white, and every other color to black, with as little "blockage" as possible?

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