I am using the following mods related to the bug:
Color Picker 0.3.0
Color Picker - Locomotive 0.2.1
Color Picker - Player 0.2.1
Color Picker - Train Stops 0.1.1
Color Picker - Wagon 0.2.1

When loading a 0.15.25 map in 0.15.27 I get the following error:

Attempt to remote call outside of an event.
stack traceback:
[]__locomotive-color__/control.lua:107: in main chunk

(the [] is actually a box)
When disabing only the locomotive mod, I get a similar error for the player. When disabling all but Color Picker itself OR when only upgrading to 0.15.26, the mod still works fine.

The 0.15.27 update states:

"Fixed that mods could do remote calls outside of events when the game isn't in a valid state."

A change in 0.15.27 brokes the sub-mods of Color Picker (Locomotive, Player, Train Stop, Wagon). Please delete them as they are going to be combined in the up coming version of Color Picker.

Hurray .. now I can patch Factorio to 0.15.28 without cringing :D