A J4100 FLC NEARLY COLLIDES WITH AN OPPOSITE DIRECTION MAULE FLYING 200 FT ABOVE AT 1 MI. THE FLYING FO HAD DECIDED TO CLB AFTER MISINTERPING THE OTHER ACFT'S ALT AS BEING BELOW THEIRS 3 MI E OF AML; VA.

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UPON ARR INTO WASHINGTON DULLES TFC AREA; MY FLT WAS ASSIGNED TO FLY THE BARIN TRANSITION TO THE N SIDE OF THE ARPT. THIS IS A STANDARD PROC. THE CAPT AND I ACKNOWLEDGED A TCASI ALERT OVER THE RADIO SYS FROM ANOTHER AIRPLANE. WE THEN HEARD A PLT ASK ATC TO REPEAT HIS INSTRUCTIONS TO HIM BECAUSE 'HE HAD THE TCASI GOING OFF IN THE OTHER EAR.' ABOUT 1 MIN LATER; OUR TCASI ALERTED US OF TFC AT EXACTLY 12 O'CLOCK; 1 MI; AND MINUS 200 FT. THE CAPT AND I VERIFIED THE TFC WAS BELOW AND I STARTED A CLB. I THEN WAITED FOR THE CAPT TO QUERY ATC OF THE CONFLICTING TFC. BUT HE DID NOT QUERY ATC. ATC THEN COMMANDED US TO DSND IMMEDIATELY BECAUSE THE TFC WAS ABOVE US! I DSNDED. I CAME TO REALIZE THAT THE EFIS SYS WAS BLOCKING THE VERT PORTION OF THE POSITIVE (PLUS) SIGN ON THE TCASI SYS. BOTH LNAV AND LOC NEEDLES COLORED GREEN AND RED RESPECTIVELY OVERPWRED AND COVERED THE AMBER COLORED TCASI ALERT AND THE POSITIVE ALT INDICATION ON THE TCASI SYS. BOTH LNAV AND LOC NEEDLES ON THE EFIS SYS WERE IN THE VERT POS. THIS VERT POS OF THE NEEDLES COVERED THE VERT PORTION OF THE POSITIVE SYMBOL OF THE CONFLICTING TFC. WHAT APPEARED TO BE A NEGATIVE INDICATION TO BOTH PLTS WAS ACTUALLY A POSITIVE INDICATION.