If you're talking about color of the Clock, you can do that by launching Settings > Lock Screen > Clock Style > Lock Screen. Tap on Color Tab. You can then choose from various color presets or set any color you like. If you're talking about other stuff in the lock screen, their color can't be changed with the settings provided by Samsung.
I can change the clocks color. But I want everything to be white for uniformity. It was possible before this beta though. Can't do it in the current version. 😞
And are all other fonts like notifications and 'use fingerprint to unlock' dark on your phone too? If so, how would it be visible on phones using a black wallpaper?