Question

if a convex lens is covered by two different colour papers green at top and red at
bottom then how it will affect the colour of the image​

Answer 1

When half of the convex lens is covered, say with black paper, the rays of light occurs and image will be formed as how the normal lens receives, which means that the focal length remains the same, but, only the brightness or intensity of the light will be diminished due to the covering of black paper.

Hence, If half part of a convex lens is covered, the focal length does not change, but the intensity of the image decreases.