How to change specific line color?

Hey guys, got stuck here. How can I change the specific line (grout) color on the design? Couldn’t find a solid answer from any videos. Tried styles>color>by material but it gets messy. Thanks!

Edit the texture in a photo editing program.

The texture is painted onto the individual rectangle though. It’s not a texture with lines

Share the SketchUp model file so we can see exactly what you have set up and give you direct help.

I draw my tile patterns with image editor and have the grout line 80% light so the joints are more subtle. (Not sketchup lines.) Example attached. To also have a stone pattern I would start with the texture file and draw the lines over in the image editor.
tile18x36rb_288pX288p_1px_gray80
This method also works better for rendering with Enscape. Otherwise I have to create double line recessed joints because Enscape ignores coplanar lines.

Looking at your screenshot suggests that you should adjust the position of the texture you are painting on the tiles so the lines fall where the grout is.
Then depending on how you have modeled the tiles, something we can’t tell if you don’t attach the model, you might be able to select and colour the grout geometry itself.