Show fee disk space?

I read that this shows the free disk space:

df -H / | awk '///{getline; print $4}'

But how do I get that to display the figure somewhere on the Touch Bar?

I worked it out but how can I change the colour of the 'button' from the dark grey to something else?
Also, the free space figure is wrong - I have 191gb free and its showing 172gb!
How do I get the accurate free space?