I want to change the color of this black line. It should have the color #7C6741
But when I do that, it looks like this
You changed the color of the fill instead of the stroke. There are many ways to change the stroke color (and add new stroke colors) instead of the fill. The main way I use is in the tools panel, clicking or double clicking on the hollow square, or in the colors panel, clicking or double clicking on the hollow square behind the filled in square. This makes the color sliders or swatches change the stroke color. Alternatively, hit X to select the stroke then edit the color.
Alternatively, you can use the eyedropper (I) to select your text to get the color, then click the curved arrows next to the fill/stroke box to swap the brown fill for the black stroke, then just set the then black fill to transparent.