While creating a simple product shot for selling one of my lenses I recently noticed a quite visible color difference between the two speedlights I have been using:

Photo of a lens with bracket lighting
The symmetric nature of this photo makes any color difference between the two speedlights quite obvious.

After this experience I wanted to examine this more closely and created a simple test setup to compare the colors of the four different speedlights I own. To do this, I placed a greycard in front of the camera and fired each flash onto this greycard with different output levels dialed in. For each output level I took 3 subsequent shots so that I could also get an insight on the consistency between shots. First things first: For each output level all tested speedlights are very consistent. So I will only focus on the remaining aspects now. This is an overview of the collected photos:

Displayed is one image per output level and per speedlight. The tested output levels were 1/64, 1/32, 1/8, 1/2. Since the Nikon and Metz speedlights have lower guide numbers, I tried to match the exposure afterwards in Lightroom. All photos are set to the same white balance using the eyedropper tool on the first image.
Displayed is one image per output level and per speedlight. The tested output levels were 1/64, 1/32, 1/8, 1/2. Since the Nikon and Metz speedlights have lower guide numbers, I tried to match the exposure afterwards in Lightroom. All photos are set to the same white balance using the eyedropper tool on the first image.

As visible from the photo grid, the two Yongnuo speedlights are quite consistent with respect to color temperature. However, the Nikon and Metz ones are a bit warmer. Moreover, it looks a bit as if their color varies with output levels. This somehow contradicts what David Hobby said

To be at least a bit more precise about the differences in color, I also include the histograms for the first photo of each speedlight. The measurement point in all cases (for the percent values Lightroom displays) was chosen in the center of the image.

Histograms for the different speedlights. From left to right: Yongnuo YN560-III, Yongnuo YN560-II, Nikon SB-700, Metz MB 44 AF-1.
Histograms for the different speedlights. From left to right: Yongnuo YN560-III, Yongnuo YN560-II, Nikon SB-700, Metz MB 44 AF-1.

At least now I know which speedlights to combine in case of a comparable product shot.