There is a grey gate, then a purple gate. All you have to do is put two items next to the gate whose colors would mix together to create the gate color. In other words, grey = white + black, so put something white and something black in the boxes next to the grey gate. Purple = red + blue, so put something red and something blue in the boxes next to the purple gate. It doesn't matter at all what you put into the boxes, as long as they are the correct colors.
- White box, black box. Red box, Blue box.