I am trying to draw an object composited by two cubes (A & B) (one on top of the other, but for now I have them a little bit more open). I am able to do it and this is the result.

(Cube A is the blue and Cube B is the one with brown text that comes from a png texture)

But I want to have any text as parameter in the cube B. I have tried what @alecnash suggested in his question, but for some reason when I try to draw cube B, cube A dissapears and everything turns purple.

This is my draw code:

public void Draw(GraphicsDevice graphicsDevice, SpriteBatch spriteBatch, Matrix viewMatrix, Matrix projectionMatrix)
        {
            graphicsDevice.BlendState = BlendState.Opaque;
            graphicsDevice.DepthStencilState = DepthStencilState.Default;
            graphicsDevice.RasterizerState = RasterizerState.CullCounterClockwise;
            graphicsDevice.SamplerStates[0] = SamplerState.LinearClamp;


            // CUBE A
            basicEffect.View = viewMatrix;
            basicEffect.Projection = projectionMatrix;
            basicEffect.World = Matrix.CreateTranslation(ModelPosition);
            basicEffect.VertexColorEnabled = true;
            foreach (EffectPass pass in basicEffect.CurrentTechnique.Passes)
            {
                pass.Apply();
                drawCUBE_TOP(graphicsDevice);
                drawCUBE_Floor(graphicsDevice);
                DrawFullSquareStripesFront(graphicsDevice, _numStrips, Color.Red, Color.Blue, _levelPercentage);
                DrawFullSquareStripesLeft(graphicsDevice, _numStrips, Color.Red, Color.Blue, _levelPercentage);
                DrawFullSquareStripesRight(graphicsDevice, _numStrips, Color.Red, Color.Blue, _levelPercentage);
                DrawFullSquareStripesBack(graphicsDevice, _numStrips, Color.Red, Color.Blue, _levelPercentage);

            }

            // CUBE B
            // Set the World matrix which defines the position of the cube
            texturedCubeEffect.World = Matrix.CreateTranslation(ModelPosition);
            // Set the View matrix which defines the camera and what it's looking at
            texturedCubeEffect.View = viewMatrix;
            // Set the Projection matrix which defines how we see the scene (Field of view)
            texturedCubeEffect.Projection = projectionMatrix;
            // Enable textures on the Cube Effect. this is necessary to texture the model
            texturedCubeEffect.TextureEnabled = true;
            Texture2D a = SpriteFontTextToTexture(graphicsDevice, spriteBatch, arialFont, "TEST ", Color.Black, Color.GhostWhite);
            texturedCubeEffect.Texture = a;
            //texturedCubeEffect.Texture = cubeTexture;
            // Enable some pretty lights
            texturedCubeEffect.EnableDefaultLighting();
            // apply the effect and render the cube
            foreach (EffectPass pass in texturedCubeEffect.CurrentTechnique.Passes)
            {
                pass.Apply();
                cubeToDraw.RenderToDevice(graphicsDevice);
            }


        }

    private Texture2D SpriteFontTextToTexture(GraphicsDevice graphicsDevice, SpriteBatch spriteBatch, SpriteFont font, string text, Color backgroundColor, Color textColor)
    {
        Vector2 Size = font.MeasureString(text);
        RenderTarget2D renderTarget = new RenderTarget2D(graphicsDevice, (int)Size.X, (int)Size.Y);
        graphicsDevice.SetRenderTarget(renderTarget);

        graphicsDevice.Clear(Color.Transparent);

        spriteBatch.Begin();
        //have to redo the ColorTexture
        //spriteBatch.Draw(ColorTexture.Create(graphicsDevice, 1024, 1024, backgroundColor), Vector2.Zero, Color.White);

        spriteBatch.DrawString(font, text, Vector2.Zero, textColor);
        spriteBatch.End();

        graphicsDevice.SetRenderTarget(null);

        return renderTarget;
    }

The way I generate texture with dynamic text is:

 Texture2D a = SpriteFontTextToTexture(graphicsDevice, spriteBatch, arialFont, "TEST ", Color.Black, Color.GhostWhite);

After commenting several parts to see what caused the problem, it seems to be located in this line

graphicsDevice.SetRenderTarget(renderTarget);