I would like to draw concentric circles like these:

by means of TikZ and foreach command. The way I achieved it is very elementary (and burdensome):

   documentclass[standalone]

   usepackage{tikz}

   begin{document}

   begin{tikzpicture}[scale=0.2]

   coordinate (O) at (0,0);

   fill[red!70] (O) circle (20);

   fill[white] (O) circle (19);

   fill[yellow!70] (O) circle (18);

   fill[white] (O) circle (17);

   fill[red!70] (O) circle (16);

   fill[white] (O) circle (15);

   fill[yellow!70] (O) circle (14);

   fill[white] (O) circle (13);

   fill[red!70] (O) circle (12);

   fill[white] (O) circle (11);

   fill[yellow!70] (O) circle (10);

   fill[white] (O) circle (9);

   fill[red!70] (O) circle (8);

   fill[white] (O) circle (7);

   fill[yellow!70] (O) circle (6);

   fill[white] (O) circle (5); 

   fill[red!70] (O) circle (4);

   fill[white] (O) circle (3);

   fill[yellow!70] (O) circle (2);

   fill[white] (O) circle (1);

   end{tikzpicture}

   end{document}

To no avail I tried to apply foreach, but this is definitely due to my lack of fluency with TikZ (I always ended up with one color, or at most two nested circle with different color). Could you be so kind and help me with these? A suggestion how to do this would be even better than the full answer.

You mean something like this:

documentclass{standalone}

usepackage{tikz}



begin{document}

begin{tikzpicture}[mystyle/.style={circle,draw,fill=none,minimum size=20, line width = 8pt}]

  foreach x in {1,3,5,7,9,11,13,15,17,19}

node [mystyle,  minimum size = x cm, color =red!70]  (2) at (0, 0) {};

  foreach x in {2,4,6,8,10,12,14,16,18, 20}

node [mystyle,  minimum size = x cm, color =yellow!50]  (2) at (0, 0) {};

end{tikzpicture}

end{document}  

which would give you:

I just made the yellow a bit lighter because, it was bleeding yellow too much :D

A SkiaSharp solution only for comparison purposes.

using SkiaSharp; // needs skiasharp nuget

using System.Diagnostics;





class ConcentricCircle

{

    static readonly SKPaint yellowStroke = new SKPaint

    {

        Style = SKPaintStyle.Stroke,

        Color = SKColors.Yellow,

        IsAntialias = true

    };





    static readonly SKPaint redStroke = new SKPaint

    {

        Style = SKPaintStyle.Stroke,

        Color = SKColors.Red,

        IsAntialias = true

    };



    static readonly float scale = SKDocument.DefaultRasterDpi / 2.54f; // dots per cm

    static readonly float width = 6 * scale; // 6 cm

    static readonly float height = 6 * scale; // 6 cm



    static float PtToCm(float pt) => pt / scale;







    public static void Generate(string filename)

    {

        yellowStroke.StrokeWidth = PtToCm(4); // 4pt

        redStroke.StrokeWidth = PtToCm(4); // 4pt



        using (var stream = new SKFileWStream($"{filename}.pdf"))

        using (var document = SKDocument.CreatePdf(stream))

        using (var canvas = document.BeginPage(width, height))

        {



            // translate first and then scale, don't reverse!

            canvas.Translate(width / 2, height / 2);

            canvas.Scale(scale);



            // draw a red circle

            for (int i = 0; i < 5; i++)

            {

                canvas.DrawCircle(0, 0, PtToCm(8) * (2 * i + 1), yellowStroke);

                canvas.DrawCircle(0, 0, PtToCm(8) * (2 * i + 2), redStroke);

            }



            document.EndPage();

        }

    }



    private static void Main()

    {

        string filename = nameof(ConcentricCircle);

        Generate(filename);



        // convert to PNG with ImageMagick

        using (Process p = new Process())

        {

            p.StartInfo.FileName = "magick";

            p.StartInfo.Arguments = $"convert -compose copy -bordercolor red -border 2x2 -density 200 -alpha remove {filename}.pdf {filename}.png";

            p.Start();

        }

    }

}

This is Raaja's solution but without the second loop (test if the count variable is odd) and with a simple draw instead of nodes (change line width to make it thicker).

documentclass{standalone}

usepackage{tikz}



begin{document}

begin{tikzpicture}[mystyle/.style={circle,draw,fill=none,minimum size=20, line width = 8pt}]

  foreach x in {1,...,20}{

    pgfmathparse{isodd(x)}ifnumpgfmathresult=1defcurrcol{red!70}elsedefcurrcol{yellow!50}fi

    draw[line width=8pt,currcol] (0,0) circle (x cm);}

end{tikzpicture}

end{document}  

A PSTricks solution.

documentclass[pstricks]{standalone}

psset{runit=4pt,unit=psrunit}

begin{document}

pspicture[linewidth=.5](-11,-11)(11,11)

    foreach i in {1,2,...,10}{%

        ifoddidefc{yellow}elsedefc{red}fi

        pscircle[linecolor=c]{i}}

endpspicture

end{document}

Note: There is an unnecessary white spot after converting to PNG.

Animated version

documentclass[pstricks]{standalone}

psset{runit=4pt,unit=psrunit}

begin{document}    

foreach j in {1,...,10}{%

pspicture[linewidth=.5](-11,-11)(11,11)

    foreach i in {1,...,j}{%

        ifoddidefc{yellow}elsedefc{red}fi

        pscircle[linecolor=c]{i}}%

endpspicture}

end{document}

A slightly different approach to the solution proposed @TeXnician (suggested by this: What is wrong with the use of `isodd` of `xifthen`?):

documentclass{standalone}

usepackage{tikz}



begin{document}

begin{tikzpicture}[mystyle/.style={circle,draw,fill=none,minimum size=20, line width = 8pt}]

    foreach x in {1,2,...,19}{

        pgfmathsetmacromycolor{isodd{numexprx}?"red!70":"yellow!50"}%

        node [mystyle,  minimum size = x cm, mycolor]  (2) at (0, 0) {};

    }%

end{tikzpicture}

end{document}