= Solution
\b[All three interactions respect <color charge>; <quark flavor> is conserved by the <strong interaction> and the <electromagnetic interaction>, but can change in the <weak interaction>.] These statements concern charges and net numbers, rather than an unchanging inventory of particles.
A <quark> transforms in the fundamental <representation> of the <color charge> <gauge group> $SU(3)_C$. The <strong interaction> can change an individual <quark>'s colour by exchanging a <gluon>, which itself carries <color charge>. For example, the colour flow $q_r\to q_b+g_{r\bar b}$ is allowed: demanding that the outgoing <quark> remain red would be incorrect. At each <interaction vertex> the full colour flow is conserved. The colourless <photon>, <W boson>, and <Z boson> instead couple to a <quark> with an identity factor $\delta_{ab}$ in colour space. A <weak charged current> can therefore change $d_r$ to $u_r$, but cannot turn $d_r$ into $u_b$ without another coloured participant. External isolated <hadrons> are <colour singlets>; <confinement> prevents direct observation of a free coloured state.
For each <quark flavor> $f$, define the <net quark flavor number> $F_f=N_f-N_{\bar f}$. The <strong interaction> and <electromagnetic interaction> preserve every $F_f$: pair creation $g\to s\bar s$, for example, changes neither net <strangeness> nor any other $F_f$. Their <quark> couplings are diagonal in <quark flavor>. The <weak charged current> contains $\bar u_i\gamma^\mu(1-\gamma^5)V_{ij}d_j$, where $V$ is the <Cabibbo-Kobayashi-Maskawa matrix>, and changes these net numbers. <Weak neutral currents> are diagonal in <tree-level Feynman diagrams>; <flavor-changing neutral currents> can occur through higher-order <Feynman diagrams>.
The observed hierarchy of <mean lifetimes> and <decay channels> supplies evidence: associated strange-particle production through the <strong interaction> preserves net <strangeness>, whereas a <kaon>'s <weak interaction> decay to <pions> changes it. The <electromagnetic interaction> process $\pi^0\to\gamma\gamma$ has zero net <quark flavor> before and after annihilation. <Neutrino oscillations> show that individual <lepton> family numbers are not exact conserved quantities. Within the usual effectively massless-<neutrino> approximation, <charged current> decays such as $\mu^-\to e^-\bar\nu_e\nu_\mu$ preserve family numbers only when the emitted <neutrinos> are counted.
Evidence for three colours includes the <hadronic R ratio> $R=3\sum_fQ_f^2$, the colour multiplicities in <W boson> <decay widths>, and <three-jet events> revealing <gluon> radiation. These test <Quantum chromodynamics> and its colour structure together; they are not direct measurements of a named red or blue charge on a free <quark>.
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