This is part of a deliberate change that will be called out in the Major Release Notes for 4.15 with more information. We now use the default value of 1 for r.TonemapperFilm instead of 0. To get the old color emissive back you'll need to change the CVar back to 0.
When multiplying vector 3 (R,G,B) values by a scalar value of increasing quantity the emissive output will begin to change colors. The most unexpected is the blue color that will turn purple while the bloom remains blue.
The color turning white while being multiplied by larger values seems intentional however, there is no documentation to be found even in our internal 4.15 release notes regarding this behavior.
Worked: Binary 4.14.3 CL 3249277
Broken: Binary 4.15 Preview 1 CL 3258144
The mesh will start blue then fade to purple if you continue it will turn white.
The mesh stays blue with increased bloom as the emissive gets multiplied by larger values.
Head over to the existingAnswerHub thread and let us know what's up.