Simple raw pixel colour changing

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I have check already in OpenCV forum, this is not duplicate question.

I want to change a colour in an image, say with [B,G,R] value [49,51,31]

I used a loop to change pixel with these intensity values to white [255,255,255]

Here's a loop in python

for row in range(r):
    for col in range(c):
        if np.all(cim[row,col] == [49,51,31]):
            cim[row,col] = [255,255,255]

It takes around 30 seconds for my current test image of resolution [1908,1429] to complete this.

I came across this post

changing all the Red values in a colour image < 255 to 255

img[img[:, :, 2] < 255, 2] = 255 using numpy fancy indexing

I tried out same for the above condition of mine

cim1[cim1[:,:]==[49,51,31]] =[255,255,255]

But I'm getting this error

cim1[cim1[:,:]==[49,51,31]] =[255,255,255]
ValueError: NumPy boolean array indexing assignment cannot assign 3 input values to the 51068 output values where the mask is true

Any solution? Anyone?