\definemathfraction[innerfrac][frac][alternative=inner,mathstyle=script,color=red]
\definemathfraction[outerfrac][frac][alternative=outer,mathstyle=script,color=blue]

\starttext

    \startformula
    a = \sqrt{b/c}
        a = \frac{b}{c} = \sqrt{\frac{b}{c}} = \sqrt{{b}\over{c}}
        a = \frac{b}{c} = \sqrt{\frac{b}{c}} = \sqrt{{b}\over{c}}
        a = \outerfrac{a}{b} = \innerfrac{a}{b} = \frac{a}{b}
    \stopformula

\stoptext
