What would be the effect of a change in the colour of incident light from violet to red on the focal length of the lens ? Also what is the name given to this kind of a defect of lens ?

When the colour of incident light changes from violet to red, the focal length of the lens increases. This is because violet light bends more than red light when passing through a lens.
As a result, violet rays focus closer to the lens, while red rays focus farther away. So, the lens has different focal lengths for different colours of light.
This defect is called chromatic aberration.

(Chromatic aberration is the inability of a lens to bring all colours of light to the same focus point, causing coloured fringes around images.)