Private pilot reported near CFIT incident during departure climbout from CNO airport.

Narrative

On straight-out departure from CNO to ZZZ tonight just after sunset; I planned to climb to 2600 ft. MSL but misread the altimeter and leveled off at 1600 ft MSL. The dim lighting condition of the dusk and the shade on the altimeter due to the downward angle of my headlight were contributing factors. It was difficult to judge my altitude above ground when it is getting dark but not completely dark. As I approached Chino Hills; my GPS terrain warning system was activated which prompted me to re-check my altimeter reading and realized I was 1000 ft. below my planned altitude. I started climb out immediately and cleared the hills at 2500 ft. MSL. Although I may have stayed legal in this process by initiating the climb early enough before the rising terrain; the highest terrain in that area is ~1500 ft. MSL and I could have been involved in an accident if the error in my interpretation of the altimeter reading was not corrected promptly. In addition to cross checking instruments and pilot carefulness; an altitude check from the ATC (in this case the CNO Tower) acknowledging 'altitude 1600 ft.; frequency changed approved' if workload permits at dusk and night may be an additional preventative measure.