for code, (color, label) in attribute_sym.items():
    sym = QgsSymbolV2.defaultSymbol(layer.geometryType())
    sym.setColor(QColor(color))
    category = QgsRendererCategoryV2(code, sym, label)
    categories.append(category)

self.field = self.dockwidget.comboBox.currentText()
renderer = QgsCategorizedSymbolRendererV2(self.field, categories)
layer.setRendererV2(renderer)
QgsMapLayerRegistry.instance().addMapLayer(layer)

defStyle = QgsStyleV2().defaultStyle()
defaultColorRampNames = defStyle.colorRampNames()
print defaultColorRampNames

# The print command returns this list:
# [u'Blues', u'BrBG', u'BuGn', u'BuPu', u'GnBu', u'Greens', u'Greys', u'Inferno', u'Magma', u'OrRd', u'Oranges', u'PRGn', u'PiYG', u'Plasma', u'PuBu', u'PuBuGn', u'PuOr', u'PuRd', u'Purples', u'RdBu', u'RdGy', u'RdPu', u'RdYlBu', u'RdYlGn', u'Reds', u'Spectral', u'Viridis', u'YlGn', u'YlGnBu', u'YlOrBr', u'YlOrRd']

ramp = defStyle.colorRamp(defaultColorRampNames[0]) # 0 is the index for the 'Blues' ramp

defStyle = QgsStyleV2().defaultStyle()
defaultColorRampNames = defStyle.colorRampNames()
ramp = defStyle.colorRamp(defaultColorRampNames[0])

for code, (color, label) in attribute_sym.items():
    sym = QgsSymbolV2.defaultSymbol(layer.geometryType())
    category = QgsRendererCategoryV2(code, sym, label)
    categories.append(category)

self.field = self.dockwidget.comboBox.currentText()
renderer = QgsCategorizedSymbolRendererV2(self.field, categories)
renderer.updateColorRamp(ramp) #update the color ramp before setting the renderer
layer.setRendererV2(renderer)
QgsMapLayerRegistry.instance().addMapLayer(layer)