The lamp in a front projector will shift in color over time more than any other display.  To keep the color performance up to the standards it was left in after a professional calibration you can touch up the color of white yourself.  I do this about every 200 hours on my projector. 

I would download the free HCFR software for the Mac and Windows and use one of the instruments it supports to do this.  I recommend the XRite DTP-94 which is available here for $150 for a low cost solution.  Be sure to store the sensor in a sealed box with desiccant to prevent water from degrading the instrument.

With the software loaded on your computer position the sensor in the light stream from the projector about one quarter of the way from the projector to the screen.  Measure a 50 to 75% level white window from Avia or Digital Video Essentials soon after the initial calibration. Record the xy color values for white, but not the Y light output measurement.  This will be your reference color for white when you recalibrate the projector in the future.  Your projector and screen will have been calibrated for x=0.3129 and y=0.3290, but these tools will not read those values because they are not as accurate as the Photo Research PR-670 I would use.

When you need to adjust the color of white in the future use the same test pattern and instrument and adjust the projector to obtain the x and y values previously measured by adjusting the white balance gains (drives) for red, green and blue or the x and y values in the projector depending on the manufacturer and model.  I would not touch the biases (offsets) in the white balance menu because they are too difficult to set and measure.  Increasing green will mostly increase y.  Increasing red will mostly increase x.  Increasing blue will decrease x and y.  The software does include some data that indicates the relative mix of red, green and blue that can help you in this process.  Also, be sure to record your starting values.  Check that you have not distorted the white balance at other levels by looking at a gray step or ramp from 0 to 100 percent.  This test pattern should look neutral black and white across the board if this is done properly.  You should find that blue and green will need to be increased as the lamp ages.