Changing colors: what do I do wrong?

[FilipV]

Thank you all for the different answers.  It is already clear to me there are several ways to reach Rome in GIMP.
I managed to get the color to be changed, but in all honesty, I didn't really know how I did it.
I suppose I need some serious  (class room - I'm old fashioned) tuition what GIMP is concerned.  I keep on trying.

Cheerio!