self.buttons = []
        for i in xrange(self.HeightOfField):
            l=[]
            for j in xrange(self.WidthOfField):
                b=QtGui.QPushButton()
                b.setFixedSize(40,30)
                l.append(b)
                self.gridLayout.addWidget(b, i, j)
                self.gridLayout.setColumnMinimumWidth(j, 40)
            self.buttons.append(l)
            self.gridLayout.setRowMinimumHeight(i, 26)

#так
b.setStyleSheet("background-color: lightblue")
#или так
b.setStyleSheet("background-color: rgb(135, 206, 250)")