One gas bleaches the colour of the flowers by reduction while the other by oxidation in the presence of moisture. The gases are

Sulphur dioxide and chlorine gas are the two gases that bleach the colour of flower by oxidation and reduction.

Chlorine reacts with water and produces a single oxygen atom also called nascent oxygen. This nascent oxygen when combines with any colour make it colourless.

${\mathrm{Cl}}_2+{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{HCl}+\left(\mathrm{O}\right) (\mathrm{O} \mathrm{is} \mathrm{oxidizing})$

Sulphur dioxide when reacts with coloured substances it releases oxygen from the substance resulting in loss of colour.

${\mathrm{SO}}_2+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{SO}}_4+2\left(\mathrm{H}\right) (\mathrm{H} \mathrm{is} \mathrm{reducing})$