Colour
SC 1.4.1 - What does "Use of Colour" mean?
An input that turns red when something is wrong. A green dot for available and a red one for taken. Links that differ from the surrounding text by colour alone.
SC 1.4.11 - What does "Non-text Contrast" mean?
You are standing outside a museum in bright sun, trying to buy a ticket on your phone. The form is there somewhere. The input fields have a pale grey outline you cannot make out, and the button blends into the background. You tilt the screen, cup your hand over it, give up, and join the queue at the desk.
SC 1.4.3 - What does "Contrast (Minimum)" mean?
Light grey text on a white background looks calm and modern. Put a bit of sunlight on the screen and it is gone. For visitors with reduced vision it is not gone occasionally, it is gone always. That is why WCAG says: text needs enough contrast with its background.