If I understand the issue, all you need to do is take the polygonal lasso, make the edges like you would using a jigsaw, make a new layer on top of the bg, then fill that layer with the same color as the current sign.
Sample the color. You can select the brush tool, double click on the top color, move you mouse to the color you wish to sample, or you can use the info tool to select the color, then when you hover the mosue and click it will change that color to the one you want. I would say choose pure black but it probably is a shade of black instead.
Alternatively, you could use a 100% hard, black brush and just paint within the lines as parents advise their creative kids

))
To make a perfect replic of the sign shape you can use the pen tool. For this kind of tool, you'll need a tutorial even though it is basic pen tool use. Like Sam, I have to go and can't look up a tut, but I will later if someone else doesn't/. Or you could use one of the above methods.
As Sam said, pick out the font you want (plenty of sites online to choose one, if it isn't already in the default fonts, but there are plenty default fonts, all the regular ones you could want really.