Dear Rohit
 
They will still contain the chlorophyll a and b but they will be broken down and there is a much larger concentration of chromoplasts, which synthesize and store pigments such as orange carotenes, yellow xanthophylls and various other red pigments; as such, their colour varies depending on what pigment they contain. 
For example in Autumn the leaves contain modified chlorophyll a and they also contain chlorophyll b and β-carotenoids. The chlorophyll pigments both are broken down and this leads to a higher concentration of β-carotenoids which give the leaves their distinct yellow/orange colour. The β carotenoids absorb blue/violet (the complementary colours) wavelengths of light and reflect yellow/orange light.
 
Regards
Arun (askIITians forum expert)
						

							
The yellow leaf contain a and b chlorophyll but due to ageing that pigments are broken down and form chromoplast like beta-carotenoid,xanthophyll etc. This type of pigment absorbs their complementory colour which may not be in effective photosynthasis range.So photosynthasis process may hamper.