How to correctly convert RGB images into nifty format?

I have RGB images which I'm loading as a numpy array. I would like to convert these images into the nifty format, which I can open using ITK-SNAP (http://www.itksnap.org/pmwiki/pmwiki.php).

Here is what I have tried to do:

import nibabel as nib 
import numpy as np

x = load_jpg_image(filename='input.jpg')  # --> x is a numpy array containing the RGB image with shape (128, 128, 3) 
img = nib.Nifti1Image(x, eye(4)) 
nib.save(img, filename='output.nii')

However, ITK-SNAP interprets output.nii as a 3D grayscale volume rather than an RGB image. To me, it seems that ITK-SNAP should be able to handle RGB data (see this); however, I don't understand how I should save img to make this possible. I'm using ITK-SNAP 3.6.0.

Solution

It seems you can create RGB images by casting them in a custom dtype:

import nibabel as nib 
import numpy as np

RGB_DTYPE = np.dtype([('R', 'u1'), ('G', 'u1'), ('B', 'u1')])

x = load_jpg_image(filename='input.jpg')  # --> x is a numpy array containing the RGB image with shape (128, 128, 3) 

# cast to custom type:
x = x.copy().view(dtype=RGB_DTYPE)  # copy used to force fresh internal structure

img = nib.Nifti1Image(x, eye(4)) 
nib.save(img, filename='output.nii')

ITK-SNAP can handle this type of image by right-clicking the image name on the left panel and selecting the option: Multi-Component Display -> RGB.