Why is Venus's sky orange-red?

Well, Rayleigh scattering has an effect. To explain it simply, it doesn't allow high energy light, but lets low light through. Orange is slightly more energetic than red, but less energetic than, say, blue. read more

Partly because of Rayleigh scattering. The atmosphere of Venus is nearly 96 times more massive than our atmosphere. With thick sulphuric acid clouds and CO2, the atmosphere scatters very efficiently almost all of light it receives. So, blue colour and green colour might get scattered out. Red and reddish colours remain. Thus the sky appears orange-red. read more

Here is a quote describing the sky of Venus: The atmosphere of Venus is so thick that the Sun is not distinguishable in the daytime sky, and the stars are invisible at night. Color images taken by the Soviet Venera probes suggest that the sky on Venus is orange-red. read more

And this is why the spacecraft image Venus with different wavelengths of light. Once you look at Venus with ultraviolet light, you can see some of the different structures in the clouds that surround Venus. read more