Does anybody know a paper or sorts in which the term "colour" charge in QCD was introduced first? Or any other source in which this label was proposed?
O.W. Greenberg is most often credited with the introduction of colour . Note the date: it was almost immediately after Gell-Mann and Zweig introduced quarks. However his approach was formally completely different as he postulated that quarks do not obey the Fermi statistics but instead, a generalisation of it, the parafermion statistics of order 3. In fact, it turns out that Greenberg wrote an essay on the history of the discover of colour . In this article Greenberg credits Han and Nambu :
Han and Nambu  were the first who explicitly introduced the color $SU(3)_c$ symmetry which is implicit in the parastatistics model. One of their motivations was to avoid fractional quark charges, so they arranged for their three flavor triplets to have different electric charges : (1, 0, 0); (1, 0, 0); (0, −1, −1) so that they are distinguishable. Averaging over the charges for each flavor gives the fractional charges of the original quark model. They proposed that the forces between these quarks would be mediated by the exchange of an $SU(3)_c$-octet of gauge vector mesons, thus giving the hidden color degree of freedom a dynamical role, and showed that such forces would make the $SU(3)_c$-singlets be the ground-state particles, to be identified with the known baryons and mesons.
As it should be clear, the charges are not right! But a few paragraphs later, Greenberg goes on to say that
While preparing for this session, I looked into the origin of the use of the word “color” for the gauged $SU(3)_c$ degree of freedom. Bram Pais , in a discussion in the Erice summer school of 1965, was the first to use color in this way. Donald Lichtenberg  also used color with this meaning in his book published in 1970. Color in this sense came into general use following the articles by Gell-Mann and by William Bardeen, Harald Fritzsch and Gell-Mann .
A work that H. Fritzsch, M. Gell-Mann And H. Leutwyler further consolidated  in the article I cited in an earlier version of this answer.
 O.W. Greenberg: Spin and Unitary Spin Independence in a Paraquark Model of Baryons and Mesons. Phys. Rev. Lett. 13, 598-602 (1964)
 Color, from baryon spectroscopy to QCD, arXiv:hep-ph/9301207
 Y. Nambu, in Preludes in Theoretical Physics, ed. A. de Shalit, H. Feshbach and L. Van Hove (North Holland, Amsterdam, 1966), p. 133; M.Y. Han and Y. Nambu, Phys. Rev. 139 (1965) B1006. A. Tavkhelidze, in High-Energy Physics and Elementary Particles (Int. Atomic Energy Agency, Vienna, 1965), also suggested three different triplets with integral charges, but did not suggest the gauge interaction.
 A. Pais, in Recent Developments in Particle Symmetries, ed. A. Zichichi (Aca- demic, New York, 1966) p. 406.
 D.B. Lichtenberg, Unitary Symmetry and Elementary Particles (Academic, New York, 1970) p. 227.
 M. Gell-Mann, Elementary Particle Physics, ed. P. Urban (Springer, Vienna, 1972); Acta Phys. Austriaca Supp. 9 (1972) 733; W.A. Bardeen, H. Fritzsch and M. Gell-Mann, in Scale and Conformal Symmetry in Hadron Physics, ed. R. Gatto (Wiley, New York, 1973) p 139.
 H. Fritzsch, M. Gell-Mann, and H. Leutwyler. Advantages of the color octet gluon picture. Physics Letters B, 47(4):365 – 368, 1973.