Is there a quark conservation law? The section on particle interactions in my revision guide says that only the weak interaction can change quark types, e.g. when a neutron changes to a proton the down quarks in the neutron are changed to up quarks.
So I was wondering: If it is only the weak interaction that can change quark type, that means that quarks can't be changed in a strong interaction. This means that the quark types have to balance on both sides of the equation in a strong interaction.
My real question: Is this therefore a valid conservation law for particle interactions?
 A: Yes, there are the quantum numbers Charm, Strangeness, Topness and Bottomness, which are conserved by strong and electromagnetic interactions, but not by weak interactions. Upness and Downness are simply the Isospin, which is also preserved for strong interactions, when the quark masses can be neglected, which is usually a very good approximation as $m_u,m_d\ll \Lambda_{QCD}$. This $SU(2)$ isospin symmetry essentially breaks into $U(1)^2$, upness and downness, when the light quark masses become relevant.
A: Is there a quark conservation law?
No. In proton-antiproton annihilation (see Wikipedia) the quarks are destroyed. See this section:
"...when a proton encounters an antiproton, one of its constituent valence quarks may annihilate with an antiquark, while the remaining quarks and antiquarks will undergo rearrangement into a number of mesons (mostly pions and kaons), which will fly away from the annihilation point. The newly created mesons are unstable, and will decay in a series of reactions that ultimately produce nothing but gamma rays, electrons, positrons, and neutrinos."
One can always talk about some interactions conserving quarks and/or certain quantum numbers, but IMHO it's going too far to start talking about quark conservation. Energy is conserved, matter isn't. 
