Could you post the code you are using to visualize the image?
A single channel image should not have any color, but the library might use some colormap to plot it.
The same applies e.g. for matplotlib and a float image:
import matplotlib.pyplot as plt
import numpy as np

img = np.random.randn(100, 100)
plt.imshow(img)  # Colored output

# plt.imshow(img, cmap='gray')  # Gray output