Just updated to the latest version of ATP and received this error. Fresh install of Factorio 2.1.7 (and a handful of other updated mods).
EDIT: fails when opening my autosave; doesn't throw the error on a new game start.

Error while applying migration: Automatic Train Painter: 0.3.0-update.lua

Automatic_Train_Painter 2.1.0 errored when running a required file.

Automatic_Train_Painter/migrations/0.3.0-update.lua:8: attempt to index field 'manual-color-module' (a nil value)
stack traceback:
Automatic_Train_Painter/migrations/0.3.0-update.lua:8: in main chunk

Thanks for the report -- I've replaced the 2.1.0 release with a version that (hopefully) includes a fix. Please let me know if this issue persists.

Had to delete and re-install the mod but, yep, looks good now. Thank you, love your work!

Phew, great to hear. Thanks for using this mod!