You can configure the LCD contrast on Cisco SPA500 series IP phones.
To configure the IP phone display contrast on the phone:
Cisco SPA5XXG
Step 1 Click Admin Login > advanced > User.
Step 2 In LCD, in the LCD Contrast field, enter a numeric value from 1 to 30. The higher the number, the higher the contrast on the IP Phone display.
Step 3 Click submit all changes.