Narrative:

After an uneventful night landing; I was instructed by tower to exit the runway by turning right on taxiway K3. I did not notice the yellow runway/taxiway marking that showed the path to follow to exit the runway onto the centerline of the taxiway. I saw the blue taxiway edge lights but misinterpreted them as centerline lights; so in making the turn from the runway onto the taxiway; I was too far to the right and the nose wheel and main wheel of my right float went off the pavement into the grass. I kept moving and with power I was able quickly to get back onto pavement. I do not know whether I caused damage to any of the taxiway edge lights. I taxied uneventfully to the ramp. I am not aware of any damage to the aircraft. I know that taxiway edge lights are blue whereas taxiway centerline lights when present are green. I simply screwed up. I believe fatigue was a big factor here. I had [been flying the entire previous day]. So at the time I landed at ZZZ I had been up for 18 1/2 hours and had been doing some challenging (for me) flying that day. By the time I landed I was tired and did not do what in retrospect I should have done: stop; sort out the situation; compare what I was seeing with what I know about runway lights and markings; then correct my taxi direction to the centerline. I believe fatigue was the main cause of my error; but in addition I allowed an external factor pressure me to depart early; as planned; instead of delaying my departure to get more sleep.

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Original NASA ASRS Text

Title: Amphibious aircraft pilot reported a taxiway excursion at night due to fatigue and misinterpretation of visual cues.

Narrative: After an uneventful night landing; I was instructed by Tower to exit the runway by turning right on taxiway K3. I did not notice the yellow runway/taxiway marking that showed the path to follow to exit the runway onto the centerline of the taxiway. I saw the blue taxiway edge lights but misinterpreted them as centerline lights; so in making the turn from the runway onto the taxiway; I was too far to the right and the nose wheel and main wheel of my right float went off the pavement into the grass. I kept moving and with power I was able quickly to get back onto pavement. I do not know whether I caused damage to any of the taxiway edge lights. I taxied uneventfully to the ramp. I am not aware of any damage to the aircraft. I know that taxiway edge lights are blue whereas taxiway centerline lights when present are green. I simply screwed up. I believe fatigue was a big factor here. I had [been flying the entire previous day]. So at the time I landed at ZZZ I had been up for 18 1/2 hours and had been doing some challenging (for me) flying that day. By the time I landed I was tired and did not do what in retrospect I should have done: stop; sort out the situation; compare what I was seeing with what I know about runway lights and markings; then correct my taxi direction to the centerline. I believe fatigue was the main cause of my error; but in addition I allowed an external factor pressure me to depart early; as planned; instead of delaying my departure to get more sleep.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.