Thank you so much,
Actually, after upgrade to version 1.5 in Windows, it seems that what didn't work before now does :
change color or other attributes do now work under section "frame content editiing"
(I can't be sure exactly if taht's its name, I've got the French version of Scribus)
One can highlight and modify what is highlighted only in that " Frame Content Editor" which makes sense,
insofar as zooming focus on text is much easier there.
Scribus is going to let me forget about other progs, after some training, of course!