\def\oklepaj#1{\left(#1\right)}
\def\t{\dot N(t)=
  A\oklepaj{\exp\oklepaj{-\frac{t-t_0}{τ_{\text{eksp.}}}}-\exp
    \oklepaj{-\frac{t-t_0}{τ_{\text{fast}}}}}+
  B\oklepaj{\exp\oklepaj{-\frac{t-t_0}{τ_{\text{eksp.}}}}-\exp
    \oklepaj{-\frac{t-t_0}{τ_{\text{slow}}}}}}
\def\T{Test}

\starttext
\doifmode{textext}{Mode: textext}
\doifmode{sometxt}{Mode: \letterbackslash sometxt}

\startmode [textext]
  \startMPcode
%   label(textext("$\t$\&"),origin); % doesn't work
    label(textext("$\t$\letterbackslash\&"),origin);
  \stopMPcode
\stopmode
\startmode [sometxt]
  \startMPcode
%   label(\sometxt{$\t$\&},origin); % doesn't work, either
    label(\sometxt{$\t$\letterbackslash\&},origin);
  \stopMPcode
\stopmode

\switchtobodyfont [sans, 36pt] \bf

\startmode [textext]
\startMPcode
for i=0 upto 100:
  label(textext("\colored [b=.7, a=1, t=.3]{\T}"),(i*1mm,0cm))
    rotatedaround ((i*1mm,0cm),i*2);
  label(textext("\colored [g=.7, a=1, t=.2]{\T}"),(i*1.1mm,0cm))
    rotatedaround ((i*1.1mm,0cm),i*2);
endfor;
\stopMPcode
\stopmode

\startmode [sometxt]
\startMPcode
for i=0 upto 100:
  label(\sometxt{\colored [b=.7, a=1, t=.3]{\T}},(i*1mm,0cm))
    rotatedaround ((i*1mm,0cm),i*2);
  label(\sometxt{\colored [g=.7, a=1, t=.2]{\T}},(i*1.1mm,0cm))
    rotatedaround ((i*1.1mm,0cm),i*2);
endfor;
\stopMPcode
\stopmode
\stoptext