What quantum numbers characterize a 1 particle state? Single particle states in QFT are defined to be irreducible invariant subspaces under the action of the Poincare group.  Such states are generally specified by a spin projection and a momentum.  Is it possible for a 1 particle state to have other quantum numbers?  
My expectation is no, as existence of any other degrees of freedom would suggest that the state is not irreducible.
 A: Hint: colour, electric charge, etc. (keyword: internal symmetry).
Recall that in QM we always label the states of the theory by simultaneously diagonalising a complete set of commuting operators, typically the Hamiltonian and a maximal set of commuting symmetries (conserved charges). These commuting operators become labels on states.
One must therefore ask oneself what is the most general set of symmetries allowed in a certain QFT, and here Coleman-Mandula is essential: this theorem guarantees that the most general set is given by the direct product of Poincaré times a compact Lie algebra. These symmetries are dubbed external and internal respectively.
As these two sets don't mix (they commute), you can diagonalise them independently, and so you can perform the standard Wigner classification, so you end up with 1PS labelled by external quantum numbers: mass, three-momentum, spin and helicity. No other external quantum numbers exist (unless we consider higher dimensional theories, where there are more than one spin quantum number). Internal symmetries give you additional quantum numbers such as electric charge, colour, etc., but OP, as most people do, does no consider these quantum numbers to label different states for a certain 1PS, but different 1PS altogether. In other words, for a fixed 1PS (fixed internal quantum numbers), the only labels are external (mass, spin, etc.).
Note that CM is crucial here: without it it would not be possible in general to split the symmetries into two groups (external and internal). For example, in QFT's with Galilean symmetries, a boost and a translation is in a sense equivalent to a phase rotation, so the three-momentum label is not completely independent from the electric charge quantum number. In general you can have very exotic mixing of internal and external symmetries, and therefore the whole classification of states can be arbitrarily different from what one expects based on the standard relativistic classification.
