What does H2SO4 do to YAS?
Ammonium sulphide is $\ce{(NH_4)_2S}$; it is colorless. Yellow ammonium sulphide contains some sulfur $\ce{S}$ dissolved in the preceding solution. So the anion  $\ce{ S^{2-}}$ is mixed with some disulphide $\ce{S_2^{2-}}$, which is yellow. If you add an acid to this solution, you produce the reaction $$\ce{S_2^{2-} + 2 H^+ -&gt; H_2S_2}$$ or$$\ce{S_2^{2-} + 2 H_3O^+ -&gt; H_2S_2  + H_2O}$$ 
But $\ce{H_2S_2}$ is thermodynamically unstable with respect to disproportionation. It gets slowly decomposed according to $$\ce{H_2S_2 -&gt;  H_2S + S}$$The sulfur S produced here is not soluble into water, and will make the solution cloudy. If there is enough disulphide, the sulphur will make a precipitate of elementary sulphur, which may be pale yellow or white.  

Ref.: N.N. Greenwood A. Earnshaw, Chemistry of the Elements, A. Wheaton &amp; Co, Exeter, 1986, p.805 - 807.