A colorless lead salt, when heated, produces a yellow residue and brown fumes. The brown fumes is of the compound [[1]].

Lead nitrate is a colorless salt. When heated it produces lead oxide which is yellow in color and nitrogen dioxide gas which is brown in color.

The reaction is as follows;

${\mathrm{PbNO}}_3\stackrel{∆}{\to }\mathrm{PbO}+{\mathrm{NO}}_2$

Therefore the answer is nitrogen dioxide.