TAXIING IN; A B737 CREW BECOME CONFUSED WITH TXWY MARKINGS AND DEPART THE TXWY FOR A SHORT DISTANCE GOING TO THE RAMP.

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WE LANDED ON RWY 25R AND CLRED THE RWY AT TXWY A5. WE THEN TAXIED E ON TXWY B. GND CLRED US TO TAXI TO OUR GATE. I INTENDED TO USE TXWY B4 TO ENTER THE RAMP AREA; BUT INADVERTENTLY TURNED SHORT OF TXWY B4 AND CROSSED THE HARD DIRT SURFACE ON THE WAY TO THE RAMP. THE LIGHTING (EVERYTHING LOOKED GRAY) AT THAT TIME (DUSK) AND THE YELLOW LINES WERE DIFFICULT TO SEE. COMPOUNDING THE ISSUE IS AN ASPHALT PATCH THAT CREATED THE OPTICAL ILLUSION THAT THE PATH WE TOOK WAS TXWY B4. THE DARK ASPHALT PATCH CUTS DIAGONALLY ACROSS LIGHTER GRAY ASPHALT; AND AT THAT TIME OF DAY/NIGHT MADE IT APPEAR TO BE THE TXWY. ABOUT 1 SECOND PRIOR TO GOING INTO THE DIRT AREA; THE FO CALLED OUT; 'HEY IS THAT A DITCH?' HOWEVER; AT THAT TIME IT WAS TOO LATE TO STOP. THE RIDE THROUGH THE DIRT WAS UNEVENTFUL AND APPEARED TO CAUSE NO DAMAGE TO THE ACFT. AT THE GATE I HAD THE ACFT INSPECTED AND HAD THE VOICE AND FDR'S PULLED. THE INSPECTION BY MAINT IDENTED NO DAMAGE TO THE ACFT. I SPOKE WITH CHIEF PLT ON CALL AND HE CLRED THE FO AND I TO CONTINUE ON OUR PAIRING. AS A 5 YR CAPT; I'M OBVIOUSLY QUITE FAMILIAR WITH FLYING INTO AND OUT OF LAS. I SUSPECT MY FAMILIARITY; VISUAL ILLUSION; AND NOT FOLLOWING A YELLOW LINE LED TO THE INCIDENT. I FEEL VERY LUCKY THAT NO DAMAGE WAS DONE TO THE ACFT; AND MY DUE DILIGENCE IN FOLLOWING A YELLOW LINE WILL BE MUCH HIGHER IN THE FUTURE. SUPPLEMENTAL INFO FROM ACN 592195: AFTER LNDG AT LAS ON RWY 25R; WE TAXIED VIA A5 TO GO TO GATE. AS I COMPLETED MY AFTER LNDG FLOW; I LOOKED OUTSIDE AND SAW 2 BLUE TAXI LIGHTS ON L AND 2 BLUE TAXI LIGHTS ON THE R. THE COLOR OF THE LIGHTING (RAMP) AND THE SUN JUST GOING DOWN MADE ALL SURFACES LOOK THE SAME COLOR. AS WE APCHED BTWN TXWY B5 AND TXWY B4; I SAW A STEEP DOWNHILL THAT DID NOT LOOK RIGHT AND STATED 'IS THAT A DITCH?' BY THE TIME I FINISHED THE STATEMENT; WE WERE DOWN THE HILL AND BEGINNING TO CLR ON THE OTHER SIDE ONTO TXWY C AND THE RAMP. WE CONTACTED MAINT UPON ARR AT THE GATE AND THEY COMPLETED AN INSPECTION OF THE ACFT AND FOUND NO DAMAGE. FACTORS LEADING TO INCIDENT: LIGHTING OF AREA CHANGE TO NORM (LNDG ON RWY 25R INSTEAD OF RWY 25L AND DIFFERENT TURNOFF AREA INABILITY TO DIFFERENTIATE COLOR OF TXWY AND DIRT TIMING OF AFTER LNDG FLOW AND TURNOFF TO TXWY TOOK FO'S ATTN FROM OUTSIDE THE ACFT. FAMILIARITY WITH THE LAS ENVIRONMENT. PLT'S ATTN TO SIT. CORRECTIVE ACTION: CHANGE CONTRAST OF TXWY VERSUS DIRT FOR BETTER VISIBILITY QUEUES.