Special thanks to Alan Ross for this explanation of the modified meter!

One of my most important tasks as a teacher is to impress upon my students that most meters, and metering systems DO NOT see colors the same way typical panchromatic film does, and that this can lead to significant exposure errors when measuring strongly colored objects or when metering through their filter of choice. Some meters see the red end of the spectrum much the way film does, but not the blue-violet end. Some other meters are exactly the opposite! The exceptions to this admonition are the Zone VI "Modified" meters, which seem to see the world pretty much the same way black-and-white film does. My Zone VI-Modified digital Pentax meter's color accuracy is so good in fact that I use a set of small filters for my meter which match the ones I put on my camera. I make readings directly through the filter on my meter and expose accordingly - no more filter factors to calculate. This also allows me to tell exactly how any given filter will affect various tones in the scene, meaning I can actually read how much more a red filter will darken a sky compared to a yellow! The modified meters are good for color film too!

Testing Your Meter's Color Response

Set up a large gray card outdoors in the sun on a clear enough day so that the sun won't be dipping in and out of the clouds every three minutes. A full 32x40 sheet of some kind of medium gray mat board is perfect. Focus on infinity and set your camera up on a tripod so that the card fills the frame. Gather the most extremely different color filters you have from opposite and in-between ends of the spectrum. Maybe include one or two particular filters you like to use a lot. These don't even have to be actual photo filters, they could be theatrical-lighting gels or other strongly colored transparent material. In my own test I used a #47B (tricolor blue), #58 (tricolor green), #25 (strong red), #15 (orange), and #12 (strong yellow).

Using the meter you want to test, take a reading of the gray card and expose exactly as the meter indicates. Take another reading through one of your filters. Place that same filter over the lens (if you were using a hand-held meter) and expose exactly as the meter indicates and note the exposure and which color it was in a notebook. Do the same for all the other colors you have handy. If you don't have a wide range of filters, you could exchange the gray card for strongly colored ones, or photograph a range of strongly colored objects, filling the frame with each object. Develop the film. Ideally, every exposure should have the same density. If any exposures, such as red for example, are significantly underexposed compared to your "no filter" exposure, then you know that your meter "sees" red as being a lot brighter than your film actually does, and you will have to compensate for this if you are forced to base your exposures on something red. If any color, blue for example, is denser (on the negative) than your "no filter" standard then you know that the meter "sees" that color as being darker than the film does and will over-expose it.