Why does the transparent color on a QQuickItem not have the same behavior as that of a Rectangle?

[Babiole]

Hello, i have transparency issue on my QQuickItem, it's color doesn't have the same behaviour of a Rectangle's color for example.
I'm on Qt 5.9.1

This is my CustomShape :

QSGNode *RoundedRectangle::updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *data)
{
    const auto r = m_fillColor.red();
    const auto g = m_fillColor.green();
    const auto b = m_fillColor.blue();
    const auto alpha = m_fillColor.alpha();
     if (!oldNode) {
		node = new QSGGeometryNode;
		geometry = new QSGGeometry(QSGGeometry::defaultAttributes_ColoredPoint2D(), nbVertice);
                geometry->setDrawingMode(QSGGeometry::DrawTriangleStrip);
                const auto material = new QSGVertexColorMaterial;
                material->setFlag(QSGMaterial::Blending, true);
                node->setGeometry(geometry);
                node->setMaterial(material);
                node->setFlag(QSGNode::OwnsMaterial);
	}
	else {
		node = static_cast<QSGGeometryNode *>(oldNode);
		geometry = node->geometry();
		geometry->allocate(nbVertice);
	}
	auto vertices = geometry->vertexDataAsColoredPoint2D();
//Just an exemple of setting a vertice
	vertices[++idx].set(posX, posY, r, g, b, alpha);
        ...
       return node;
}

This is my qml file :

RoundedRectangle {
        id:test
        anchors.top: canvas.top
        anchors.topMargin: margin
        anchors.left: canvas.left
        anchors.leftMargin: margin
        anchors.right: canvas.right
        anchors.rightMargin: margin
        height: 20
        width: 200
        borderWidth: 1
        radius: 5
        borderColor: "#C8E6C9"
        fillColor: "#501B5E20"
        antialiasing: true
    }

    Rectangle {
        anchors.top: test.bottom
        anchors.topMargin: margin
        anchors.left: canvas.left
        anchors.leftMargin: margin
        anchors.right: canvas.right
        anchors.rightMargin: margin
        height: 20
        width: 200
        radius: 5
        color: "#501B5E20"
    }

And this is the result :

Same color but different result (fillcolor and color on qml)
My item is the first rectangle of the picture. The basic rectangle is just below.

What I did wrong?

[Babiole]

@Babiole
I found the answer, this is because QSGVertexColorMaterial interpret color as premultiplied alpha. So you have to multiply your base color by your alpha percentage.

const auto r = m_fillColor.red() * m_fillColor.alphaF();
const auto g = m_fillColor.green() * m_fillColor.alphaF();
const auto b = m_fillColor.blue() * m_fillColor.alphaF();
const auto alpha = m_fillColor.alpha();