\documentclass[border=5mm]{standalone}
\usepackage{luamplib}
\begin{document}
\mplibtextextlabel{enable}
\begin{mplibcode}
beginfig(1);
path A, B; 
A = fullcircle scaled 2.5cm; 
B = fullcircle scaled 1.8cm shifted (1cm,0);
fill buildcycle(A,B) withcolor .8[blue,white]; 
drawarrow A; 
drawarrow B;
label.ulft(btex $A$ etex, point 3 of A);
label.urt(btex $B$ etex, point 1 of B);
label.rt(btex Where has the fill colour gone? etex, point 0 of B shifted (1cm,0));
endfig;
\end{mplibcode}
\end{document}