  By developing a `two-crystal' method for color erasure, we can broaden the
scope of chromatic interferometry to include optical photons whose frequency
difference falls outside of the 400 nm to 4500 nm wavelength range, which is
the passband of a PPLN crystal. We demonstrate this possibility experimentally,
by observing interference patterns between sources at 1064.4 nm and 1063.6 nm,
corresponding to a frequency difference of about 200 GHz.
