It appears that the change of text color is unintended and happens as a temporary side effect of manipulating the boss bar using commands.
The culprit seems to be
net.minecraft.server.CustomServerBossInfo.getFormattedName()
where the boss bar's name isn't copied before it's manipulated and thus gets mutated (MCP names).
@blobjim Here it actually only affects singleplayer (and LAN game host). The effect is reversed. On a server, network item stack serialization "fixes" the empty slots and they don't display the animation when scrolling.
Still present in 1.11-pre1
It appears that the change of text color is unintended and happens as a temporary side effect of manipulating the boss bar using commands.
The culprit seems to be
net.minecraft.server.CustomServerBossInfo.getFormattedName()where the boss bar's name isn't copied before it's manipulated and thus gets mutated (MCP names).