This paper investigates dynamic properties of 3D rainbow lightweight hollow sphere foams both numerically and experimentally. Two rainbow hollow sphere foams are designed with linearly varying sphere shell thicknesses and binder diameters for the purpose of achieving broadband vibration attenuation at low frequencies. The hollow sphere foams are modeled by the finite element method. The band structures of two rainbow hollow sphere foams are compared with that of the uniform foam. The results show that the foam with gradient binders exhibits a bandgap more than two times broader than that of the uniform foam at lower frequencies, and the gradient binders also lead to locally concentrated vibration modes at the bandgap edges, which are different from the global vibration modes of the uniform foam. On the other hand, the foam with gradient shell thickness could not generate complete bandgaps due to the introduced additional modes by the varied shell thickness. The bandgap extension could, hence, be realized with properly designed structural gradients of foams. The rainbow and uniform hollow sphere foam samples are manufactured subsequently by the additive manufacturing method and tested with a frequency response function measurement system. The experimental results verify the numerical calculation as well as prove further the effects of gradient designs on bandgap extension. The proposed rainbow hollow sphere foams could be instructive for future researchers to design lightweight acoustic/dynamic structures for broadband low frequency noise and vibration control.