Which colour channel from a TIFF image do I have to use?
From the readme of your kaggle link;

All images are provided in .tif format with 3 channels per image.
For 101 cases, 3 sequences are available, i.e. pre-contrast, FLAIR, post-contrast (in this order of channels).
For 9 cases, post-contrast sequence is missing and for 6 cases, pre-contrast sequence is missing.
Missing sequences are replaced with FLAIR sequence to make all images 3-channel.
Masks are binary, 1-channel images.
They segment FLAIR abnormality present in the FLAIR sequence (available for all cases).

So you could use all three channel.