i have a scene with a multiple (QGraphicsTextItem)s, and  i need to have control over their colors , so how to change a color of a QGraphicsTextItem ? is it possible anyway? i've been trying for 3 days until now . please help

thanks in advance
setDefaultTextColor(col) "Sets the color for unformatted text to col." The documentation is not clear about what "unformatted text" means.  I think it means: "all portions of the contents of the item that have not been styled."
The contents is a QTextDocument.
You style a part of a document using a QTextCursor.  You can't style the QTextDocument per se, only a part that is selected by a QTextCursor (but you can select the whole document.)
You can style a QTextCursor using method mergeCharFormat(QTextCharFormat)
The QTextCharFormat has methods:

foreground().setColor(QColor)  
setForeground(QBrush)
setTextOutline(QPen)

Foreground is a QBrush that paints several things including "text" (but better said: the fill of characters?)
One nuance is that certain newly constructed QBrush have (default to) QBrushStyle.NoBrush, which is transparent, even if you setColor().
how to change the color of a QGraphicsTextItem