Ionization injection triggered by short wavelength laser pulses inside a nonlinear wakefield driven by a longer wavelength laser is examined via multi-dimensional particle-in-cell simulations. We find that very bright electron beams can be generated through this two-color scheme in either collinear propagating or transverse colliding geometry. For a fixed laser intensity $I$, lasers with longer/shorter wavelength $lambda$ have larger/smaller ponderomotive potential ($propto I lambda^2$). The two color scheme utilizes this property to separate the injection process from the wakefield excitation process. Very strong wakes can be generated at relatively low laser intensities by using a longer wavelength laser driver (e.g. a $10 micrometer$ CO$_2$ laser) due to its very large ponderomotive potential. On the other hand, short wavelength laser can produce electrons with very small residual momenta ($p_perpsim a_0sim sqrt{I}lambda$) inside the wake, leading to electron beams with very small normalized emittances (tens of $ anometer$). Using particle-in-cell simulations we show that a $sim10 femtosecond$ electron beam with $sim4 picocoulomb$ of charge and a normalized emittance of $sim 50 anometer$ can be generated by combining a 10 $micrometer $ driving laser with a 400 $ anometer$ injection laser, which is an improvement of more than one order of magnitude compared to the typical results obtained when a single wavelength laser used for both the wake formation and ionization injection.
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