Yes, the hub is identical
Same 4x98 PCD and same 58.1 mm centre bore: wheels from Alfa Romeo 155 bolt on and centre on Fiat 500 (312) without rings. You still need to check the offset and tyres, detailed below.
≈: width estimated from the original tyre size.
Wheel by wheel: offset and tyres
Each original wheel from Alfa Romeo 155 is compared with the one from Fiat 500 (312) of the same diameter: how much closer the wheel gets to the shock absorber or how far it sticks out of the wheel arch, and whether the tyre keeps the same overall diameter.
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≈6Jx15 ET40 · 195/55R15 — Check on the car
Compared with the original 6Jx15 ET35 on Fiat 500 (312): shock absorber side +5 mm, wheel arch side −5 mm, tyre diameter +1.9 %.
Comparison for guidance only (estimated width). Check in detail
Before fitting the wheels
An identical hub is not enough: offset, width and diameter change the position of the wheel, and larger calipers can touch a smaller wheel. Original wheels often use bolts or nuts with a specific seat: keep the fixings intended for the wheel, with the thread of Fiat 500 (312) (M12x1.25), tightened to the torque shown on its vehicle page. Finally, check that the size is among the approved fitments for your vehicle. This comparison is for guidance only: have the fitting approved by a professional.
And the other way round: wheels from Fiat 500 (312) on Alfa Romeo 155?