Final answer:
On a traditional negative, the color of a yellow subject will appear as its complementary color, blue. This is due to the process known as color inversion, in which colors in a photograph are reversed.
Step-by-step explanation:
The color on the negative of a yellow sweater would appear blue. When a photo is taken, the film in the camera creates a negative where the colors are reversed or inverted. This process is also referred to as color inversion. Hence, light colors appear dark and vice versa. In terms of color negatives, the color wheel rules apply meaning complementary colors swap each other, so Yellow's complementary color on a color wheel is Blue. So, in your photograph's negative, the yellow sweater would appear blue.
Learn more about Color Inversion