I think that the reason for the yellow color, is because that this picture was capture during the zero current phase on the mains frequency, in combination of a very short exposure time, so only the phosphor afterglow appears.
If the phosphor of this lamp is halophosphors, this is the reason for the yellow color, as halophosphors afterglows with a yellow color for less then a millisecond after the lamp turned off, and in some cases, this yellow light even possible to seen in the naked eyes.

Best set your shutter to TV mode @ 1/50 or 1/60 second depending on your phase frequency. You should then capture a whole phase and the picture will come out more "real" especially around the cathode.