Solange Knowles is saying goodbye to her late 'Bring It On' costar with an emotional farewell to Hayden Panettiere ... remembering Hayden for courageously living her "radical truth."

Solange says Hayden was on an endless quest for vulnerability and healing ... and she laments the way Hayden finally found the peace she deserved.

The two actresses starred in "Bring It On: All or Nothing" -- the third film in the popular cheerleading franchise.

Solange says the two had "so many parallels" in their lives ... and she was immensely proud that they got to work together.

She also shared several pics of them goofing off on set.

As we reported ... Hayden died Sunday afternoon at a housing complex in Greenville, South Carolina.

Hayden's sudden death sent shockwaves through Hollywood ... resulting in a flood of posts from celebs and fans.