Reproduce bug:

import torch
import torchvision.utils as utils

num_masks, h, w = 4, 100, 100
img = torch.randint(0, 256, size=(3, h, w), dtype=torch.uint8)
masks = torch.randint(0, 2, (num_masks, h, w), dtype=torch.bool)

utils.draw_segmentation_masks(img, masks, colors="red")

which throws:

ValueError: There are more masks (4) than colors (3)

where the expected outcome is the drawing of all the segmentation masks in a single color (as stated in the docs).

Notice that, in the following settings the function works as intended;

• num_masks=3 and colors="red",
• num_masks=4 and colors="blue",
  meaning, in the case of num_masks=len(colors).

Currently, the respective tests pass even though the function does not work properly. This is because the test is checked in the presence of only 2 masks:

The Fix:
I have adapted the code in draw_segmentation_masks such that it also accepts a string of colors. The code was adapted similarly to how it is handled in draw_bounding_boxes, where the issue does not appear.
Specific tests passed with:

pytest test/test_utils.py -vvv -k test_draw_segmentation_masks