The Idea in Brief
Unlike modern English, in Koine Greek the general relative pronoun (ὅστις, ἥτις, ὅ τι) may be attracted in case and number not to its antecedent, but to its predicate nominative. That is, in the passage of Col 3:5, the relative pronoun ἥτις ("which") is feminine singular not because πλεονεξία ("greed") is feminine singular, but because the predicate nominative εἰδωλολατρία ("idolatry") is feminine singular. The reference of "idolatry" therefore captures all the sins mentioned in Col 3:5.
Discussion
The same grammatical structure appears in the following verses, where the relative pronoun is attracted not to its antecedent, but to its predicate nominative.
1 Cor 3:17 (GrNT)
17 εἴ τις τὸν ναὸν τοῦ θεοῦ φθείρει, φθερεῖ τοῦτον ὁ θεός: ὁ γὰρ ναὸς τοῦ θεοῦ ἅγιός ἐστιν, οἵτινές ἐστε ὑμεῖς.
If any man destroys the temple of God, God will destroy him, for the temple of God is holy,
and that is what you are. (NASB)
The relative pronoun is οἵτινές, which is masculine plural. However, the antecedent of this relative pronoun is ναὸς, which is masculine singular. Here the relative pronoun is attracted not to its antecedent, but instead to its predicate nominative ὑμεῖς, which is masculine plural.
Eph 3:13 (GrNT)
13 διὸ αἰτοῦμαι μὴ ἐγκακεῖν ἐν ταῖς θλίψεσίν μου ὑπὲρ ὑμῶν, ἥτις ἐστὶν δόξα ὑμῶν.
Therefore I ask you not to lose heart at my tribulations on your behalf, for they are
your glory. (NASB)
The relative pronoun is ἥτις, which is feminine singular. However, the antecedent of this relative pronoun is θλίψεσίν, which is feminine plural. Here the relative pronoun is attracted not to its antecedent, but instead to its predicate nominative δόξα, which is feminine singular.
1 Tim 3:15 (GrNT)
15 ἐὰν δὲ βραδύνω, ἵνα εἰδῇς πῶς δεῖ ἐν οἴκῳ θεοῦ ἀναστρέφεσθαι, ἥτις ἐστὶν ἐκκλησία θεοῦ ζῶντος, στῦλος καὶ ἑδραίωμα τῆς ἀληθείας.
but in case I am delayed, I write so that you will know how one ought to conduct himself
in the household of God, which is the church of the living God, the pillar and support
of the truth.
The relative pronoun is ἥτις, which is feminine singular. However, the antecedent of this relative pronoun is οἶκος, which is masculine singular. Here the relative pronoun is attracted not to its antecedent, but instead to its predicate nominative ἐκκλησία, which is feminine singular.
Conclusion
In summary, in Koine Greek the case and number of the general relative pronoun may reflect the case and number of the predicate nominative instead of its antecedent, which would be the case in modern English. So the accurate translation of this verse would be as follows, which captures all the sins mentioned in the verse:
Col 3:5
5 Therefore consider the members of your earthly body as dead to immorality, impurity, passion, evil desire, and greed, which amount to idolatry.