  We present MSX two-color diagrams that can be used to characterize
circumstellar environments of sources with good quality MSX colors in terms of
IRAS color regions for oxygen-rich stars. With these diagrams we aim to provide
a new tool that can be used to study circumstellar environments and to improve
detection rates for targeted surveys for circumstellar maser emission similar
to the IRAS two-color diagram. This new tool is especially useful for regions
in the sky where IRAS was confused, in particular in the Galactic plane and
bulge region. Unfortunately, using MSX colors alone does not allow to
distinguish between carbon-rich and oxygen-rich objects. An application of this
tool on 86 GHz SiO masers shows that for this type of masers an instantaneous
detection rate of 60% to 80% can be achieved if target sources are selected
according to MSX color (region).
  Our investigations may have revealed an error in the MSX point source catalog
version 2.3. That is, the photometry of the 21.3 $\mu$m (MSX E filter) band for
most weak 8.28 $\mu$m (or MSX A filter) band sources seems off by about a
factor two (0.5--1 magnitude too bright).
