Which of the following material is good to understand incomplete dominance?

In a cross between true red flowered (RR) and true breeding white flowered (rr), snapdragon plant, the ${\mathrm{F}}_1\left(\mathrm{Rr}\right)$ was pink. When the ${\mathrm{F}}_1$ was self pollinated the ${\mathrm{F}}_2$ resulted in the following ratio $1\left(\mathrm{RR}\right)$ red; $2\left(\mathrm{Rr}\right)$ pink; $1\left(\mathrm{rr}\right)$ white. Above condition can be explained by-