Series I (92 to mid 93) except Q4s had body colour triangle, series II (model year 1994, late 93 until April 95) all had chrome triangle. More importantly the body structure (front end and sills) was stiffened and suspension and front wheel geometric changed.
At the same time (in Denmark) a steering wheel air bag and side collision bars in the doors was fitted.
Series III (May 95 on) cars have 26 mm wider track, 2,0 16v replaced the 2,0 TS and got the 287 mm front brakes of the 2,5 and Q4, the quick steering rack and the S(port) version was introduced with 15 mm lower ride and sportier springs, black/grey 15" Speedlines (16" optional), front and side spoiler and an optional boot lid spoiler. Later the 1.8 TS and 1,7 was replaced by 1,8 16v and 1,6 16v. All 16 valve engines is of the new "Modulo" engine family used by the Fiat Group in the mid, large, coupe and spider range models – in Alfa models with a special cylinder head with 2 spark plug and variable inlet valve timing (the variable timing is available on certain Fiats and Lancias, as well, eh - sorry, giving the same problems with clatter and broken tension wheels).
I hope this clears the bewildering to newcomers to the 155 range and its variants.