esriSymbols:FillSymbol of colour visibility

Discussion created by kailun92 on Jan 6, 2014
I have this colour that shows a cricle with fading red colour. I found the colour code of #40FF2E. However I am wondering what is the 21 behind ?
<esriSymbols:FillSymbol x:Name="circle" BorderBrush="#40FF2E21" Fill="#40FF2E21"  />

I have another colour code that is
<esriSymbols:FillSymbol x:Name="circle1" BorderBrush="#ff852e21" Fill="#ff852e21"  />
It doesn't seems to be fading like the red above, but instead there is no fading effect. Can someone help me with this ?