1. Use natural light - place your flower in front of the window but place a sheet of opaque plastic (around £2 from paperchase) behind your flower - this will allow light through but effectively cut out your garden shed etc or whatever is outside your window. (Tracing paper may work just as well)
2. Set your metering to spot metering (a rectangle with a dot in) - and focus on the flower itself.
3. Over expose by +1
4. If you still have a slight grey or blue tinge I then correct this in Photoshop (am assuming you may not have this - but apps like snapseed have a lighten/brighten option)
I thought that I had taken a picture of the set up - will do next time I use it.