Question: I was trying to run a code that I saw on a answer for this [question](https://mathematica.stackexchange.com/questions/10811/how-do-i-fill-in-a-circle-made-by-parametricplot-with-one-solid-color)

The code is simply

```
ParametricPlot[{Cos[u], Sin[u]}, {u, 0, 2 Pi}] /.
  Line[l_List] :> {{Red, Polygon[l]}, {Black, Line[l]}}

```

And the result was supposed to be:

![filled circle](https://i.stack.imgur.com/JOI13.png)

However, when I run the same code in Mathematica 10.2 I get this result:
![filled circle bug](https://i.imgur.com/0ti5CXN.png)

I'm running exactly the same code, no extra global variables. Also, the color Red works perfectly fine in other situations. I've tried RGBColor[1,0,0] and the result is the same. 

Similar issues happen with other colors too. Every color is "brighter".

Answer: 
The answer is simple. Per default Mathematica reduces the opacity of the colors. Try the following:

```
ParametricPlot[{Cos[u], Sin[u]}, {u, 0, 2 Pi}] /. 
 Line[l_List] :> {{Red, Polygon[l]}, {Black, Line[l]}}

% /. _Opacity :> Opacity[1]

```

If you look at the `InputForm` of your graphics, you will notice a 

```
FaceForm[Opacity[0.3]]

```

call right in the front. With this knowledge, you can simply fix it by calling:

```
ParametricPlot[{Cos[u], Sin[u]}, {u, 0, 2 Pi}, 
  PlotStyle -> Opacity[1]] /. 
 Line[l_List] :> {{Red, Polygon[l]}, {Black, Line[l]}}

```
I think using the 2nd argument of `Opacity` is a slightly simpler way to correct the problem than the methods given by halirutan, although those are perfectly fine.

```
ParametricPlot[{Cos[u], Sin[u]}, {u, 0, 2 Pi}] /. 
  Line[l_List] :> {{Opacity[1, Red], Polygon[l]}, {Black, Line[l]}}

```

[![plot](https://i.stack.imgur.com/YOgGH.png)](https://i.stack.imgur.com/YOgGH.png)