Missing text highlighting Color when selecting it in Preview
Solution 1:
If I understand your question..
The text highlight (select) color for Preview is set in your
System Preferences > General
You can customize it by selecting Other then choose the color and how dark you want it by moving the slider.
Solution 2:
This bug also affects Microsoft PowerPoint running under Mojave. If the highlight color is set to "Graphite" you may not be able to see the text highlight and the text selection cursor will disappear. Any other highlight color seems to work.
To change the highlight color, go to System Preferences > General > Highlight Color.