最初に他のすべての色を抑制するというあなたの提案は良いものです。これを行うためのコードを次に示します。
from skimage import io, color, exposure, img_as_float
import matplotlib.pyplot as plt
# http://www.publicdomainpictures.net/view-image.php?image=26890&picture=color-wheel
image = img_as_float(io.imread('color-wheel.jpg'))
blue_lab = color.rgb2lab([[[0, 0, 1.]]])
light_blue_lab = color.rgb2lab([[[0, 1, 1.]]])
red_lab = color.rgb2lab([[[1, 0, 0.]]])
image_lab = color.rgb2lab(image)
distance_blue = color.deltaE_cmc(blue_lab, image_lab, kL=0.5, kC=0.5)
distance_light_blue = color.deltaE_cmc(light_blue_lab, image_lab, kL=0.5, kC=0.5)
distance_red = color.deltaE_cmc(red_lab, image_lab, kL=0.5, kC=0.5)
distance = distance_blue + distance_light_blue - distance_red
distance = exposure.rescale_intensity(distance)
image_blue = image.copy()
image_blue[distance > 0.3] = 0
f, (ax0, ax1, ax2) = plt.subplots(1, 3, figsize=(20, 10))
ax0.imshow(image)
ax1.imshow(distance, cmap='gray')
ax2.imshow(image_blue)
plt.show()
