${\mathrm{f}}_v\text{ and }{\mathrm{f}}_{\mathrm{r}}$ are the focal lengths of a convex lens for violet and red light respectively and ${\mathrm{F}}_{\mathrm{v}}\text{ and }{\mathrm{F}}_{\mathrm{r}}$ are the focal lengths of a concave lens for violet and red light respectively, then

According to lens makers formula

 $\frac{1}{\mathrm{f}}=(\mathrm{\mu }-1)\left(\frac{1}{{\mathrm{R}}_1}-\frac{1}{{\mathrm{R}}_2}\right)\Rightarrow \frac{1}{\mathrm{f}}\propto (\mathrm{\mu }-1)$

$\text{ since }{\mathrm{\mu }}_{\text{Red }}<{\mathrm{\mu }}_{\text{violet }}\Rightarrow {\mathrm{f}}_{\mathrm{v}}<{\mathrm{f}}_{\mathrm{r}}\text{ and }{\mathrm{F}}_{\mathrm{v}}<{\mathrm{F}}_{\mathrm{r}}$

Always keep in mind that whenever you are asked to compare (greater than or less than) u, v or f you must not apply sign conventions for comparison