Answer : They are complementary colors of light.
Explanation :
When blue and yellow light combines, it produces white light. This is because blue light and yellow light are complementary colors of light.
The pairs of colors when mixed and produce white color are called as complementary colors.
More complementary colors are :
(i) Red - Cyan
(ii) Green - Magenta
(iii) Blue - Yellow
Hence, we can say that blue light and yellow light combine to produce white light because they are complementary colors of light.