Filling with colour issue

Hi all,

basically the reason i’m posting is because I want to find out if there is an easy way of being able to fill in a space with the fill tool even though the outline may be made up of multiple layers. This has been a problem for me when it comes to colouring in, as most parts arent fully sealed because they only make up smaller elements of the outlines, which when viewed together for all the layers they make the area I want to fill in.

It happens when you are creating say a characted but you would for example make the neck by having two seperate lines instead of drawing it as a rectangle shape. The top and bottom part of the rectangle are formed by the head and the body itself so there isnt a need. This is all good and well, but if the neck is on its own layer and you want to quickly fill it in with a colour, you can’t because it is made up of two vertical lines that dont form a completed shape. I.E. ----> I I
Im wondering if there is a way to bypass this by assuming that the seperate layers are linked, therefore it would in that case link the head the neck and the body parts together to create a fillable area in the neck.

Any help would be great, cheers.