We show that fractal clusters of hollow Si nanoparticles provide both magnetic hotspots (MHs) and electric hotspots (EHs). The hollow size tailors the wavelength dependence of the field enhancement. In the wavelength window 400-750 nm, magnetic field intensity enhancements of 10-3790 and electric field intensity enhancements of 10-400 are achievable. Wavelength-tuned MHs and EHs allow better enhancement of Raman optical activity, fluorescence and circular dichroism of molecules, and so on. Si nanoparticles overcome the limitations of metallic ones, which provide only EHs at the price of heat perturbations on a nearby quantum emitter due to metallic ohmic losses.