Sure.  Assuming you're using Poser 5,6,7:  Find the material you
want to change, and the values such as Diffuse Color or Ambient Value. 
Look for the little 'key' next to each value.  Click on the key and set it to
Animated.

Then you can go to the frames where you want the change, and set
those Animated parameters to the desired value.

Note: These changes are linear, so if you want an abrupt change between
frames 30 and 31, you'll need to set the first value at frame 1, then the
first value again at frame 30 .... then set the second value at frame 31
and again at the end.

In 6 and 7, Poser does you a great favor.  After you turn the key to Animated,
Poser automatically creates a new dial for that value on the BODY of the
figure, or on the prop itself if you're dealing with a prop.  This dial can be
animated in the keygraph just like any old rotation or translation, which
means you can make it rounded (spline) instead of linear if you want,
and it's a lot easier to keep track of.

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