That is a great point, thank you! I've filed it with the team that works on components and styling.
In the meantime, you can use Custom CSS to change the color!
Here's the CSS I used:
#retool-widget-multiselectListbox1 > div > div._hPGyJ > div._qiZr9._retool-multiselectListbox1 > div > div > div > div._8uDUW > div > div > footer > div > div > button {
color: black;
}
And here's a quick guide on Custom CSS in Retool:
https://community.retool.com/t/how-to-use-custom-css-in-retool-apps/17300
Let me know if you have any questions at all!