Here is a reference to the effect. It is given in terms of light output non-linearity (gamma) but I think it could be explained in terms of electrical non-linearity giving rise to the optical effect.
Here is an interesting reference from the work done by the BBC in 1954 with NTSC colour test transmissions in London.
APPENDIX 1
INCREASE IN APPARENT BRIGHTNESS ;"¦:? CHROMINANCE SUBCARRIER DOT STRUCTURE
DUE TO NON-LINEARITY OF PICTURE TUBE TRANSFER CHARACTERISTIC
Recent tests have revealed that the brightness of those portions of the screen of a picture tube in which chrominance subcarrier is present is greater than that of a comparison patch having the same luminance but no chrominance contribution. (continues in some detail).