What mainly causes the difference between red-orange and yellow-orange?

Correct answer: The ratio of red and yellow

The governing concept is tertiary-colour formation by changing the proportions of neighbouring primary colours. Orange comes from red and yellow; increasing red produces a red-orange appearance, while increasing yellow produces yellow-orange. Thus option B is correct. Paper thickness and brush size may affect application, but they do not determine the colour relationship, and the colour of water is not the controlling pigment factor.