Is the formula used for the color - class mapping?
Here is an example how to create your own mapping:
import torch

import numpy as np
import matplotlib
import matplotlib.pyplot as plt

# Create dummy target image
nb_classes = 19 - 1 # 18 classes + background
idx = np.linspace(0., 1., nb_classes)
cmap = matplotlib.cm.get_cmap('viridis')
rgb = cmap(idx, bytes=True)[:, :3]  # Remove alpha value

h, w = 190, 100
rgb = rgb.repeat(1000, 0)
target = np.zeros((h*w, 3), dtype=np.uint8)
target[:rgb.shape[0]] = rgb
target = target.reshape(h, w, 3)

plt.imshow(target) # Each class in 10 rows

# Create mapping
# Get color codes for dataset (maybe you would have to use more than a single
# image, if it doesn't contain all classes)
target = torch.from_numpy(target)
colors = torch.unique(target.view(-1, target.size(2)), dim=0).numpy()
target = target.permute(2, 0, 1).contiguous()

mapping = {tuple(c): t for c, t in zip(colors.tolist(), range(len(colors)))}

mask = torch.empty(h, w, dtype=torch.long)
for k in mapping:
    # Get all indices for current class
    idx = (target==torch.tensor(k, dtype=torch.uint8).unsqueeze(1).unsqueeze(2))
    validx = (idx.sum(0) == 3)  # Check that all channels match
    mask[validx] = torch.tensor(mapping[k], dtype=torch.long)