el problema lo tienes aqui
.portafolio article:hover .bw-wrap, cámbialo por
.portafolio article:hover Asi seria:
Código CSS:
Ver original.portafolio article:hover{
-webkit-transition: all 300ms linear;
-moz-transition: all 300ms linear;
-ms-transition: all 300ms linear;
-o-transition: all 300ms linear;
transition: all 300ms linear;
-webkit-filter: grayscale(0);
filter: grayscale(0);
}