What is the difference between the red and blue immigration stamps I received at the Thai border?

The difference between the red and blue immigration stamps at Thai border crossings is mainly due to a change implemented in late 2019. Previously, a rectangular blue ink stamp was used, which has now been replaced with a more visually distinct 'waving flag' design that comes in either red or blue ink. The red ink stamp appears to be used more frequently, although some officers still use blue ink.