Why might a bright yellow solid form when two clear, colorless liquids are mixed?

When two substances (can be aqueous or liquids) in solution react, they yield a solid and an aqueous compound.

This is a double displacement reaction in which the solid is known as a precipitate.

You can determine which compound is the precipitate by using a solubility chart.

Soluble => #aq# compound.
Low solubility => precipitate will form #(s)#.

An example of a yellow precipitate forming is through the reaction of potassium iodide, #KI# with lead (II) nitrate, #Pb(NO_"3")_"2"#.

The products are lead (II) iodide, #PbI_"2"#, and potassium nitrate, #(KNO_"3")#.

Notice how both solutions (before reaction) are colourless. The solution in the person's hand is colourless and the solution in the other beaker is colourless (before reaction).

We can use the solubility chart to determine the precipitate.

We see that for the first product #PbI_"2"#, the anion (iodide) and cation (lead with a #2+# charge) will result in a precipitate. For #(KNO_"3")#, we see that the anion (nitrate) can react with any cation and it will form an aqueous compound.

Hope this helps :)