Photoshop :: Change Colour Of Outer Line Of A Circle?

I drew up a circle with no fill inside, to show where the location of my highway storm culvert will be.
 
I changed the thickness of the outer line of my circle shape by creating a stroke effect at layer → blending options → stroke; then I changed the thickness of the line by right clicking in stroke at the layers panel and then choosing "scale effect."
 
But the problem I have is that the outer line is gray, and I want it to be black.
 
The outer line was originally red though so I changed my image → mode to grayscale, because I couldn't find a way to change the colour while keeping my image in RGB colour.  So I've managed to make the outer line gray by changing the image mode but I still can't change the colour of the outer line to black.
 
I'm sure there must be other ways of changing the outer line colour without having to change the image mode.

The text next to the circle says «Obra de desagüe». That's spanish for "drainage construction site."
 
You see, I can't change the colour because I have no clue at where the "change colour option" is. I tried going to layer → layer properties, but in there there's only 7 colours to choose from. I chose a random colour, just to see what happens and what truly changed colour was the layer, within the layers panel; not the shape contained within the layer.