Using numbers higher than 100 in tikz color definition, when plotting from csv

Question

I'm trying to plot an image, which is basically a matrix of intensity values. I want the colour scheme to range from black for the lowest to white for the highest value, with a defined colour in the middle. (I based my approach on this answer to a similar question.)
Here is a MWE:
documentclass[tikz]{standalone}
begin{document}
    begin{tikzpicture}[scale=1]
        foreach y [count=n] in { %this is normally a VERY large matrix
            {0,50}, 
            {75,100},
} {foreach x [count=m] in y {fill[white!x!red!x!black] (1*n,-1*m) rectangle ++(1,1);}}
end{tikzpicture}
end{document}

The problem is that my values in absolute terms range from 0 to about 400 and in my current approach I need to normalise them to a range from 0 to 100. This technically works, however, this means that I loose 3/4 of my colour information. Unfortunately, the data is very sensitive and I am now looking for a way to use a larger range.
My new approach is dividing the colour scheme into two parts, so that I can use at least values from 0 to 200. However, this does not seem to work, and I am not that proficient in using if-commands in LaTeX.  (I am aware of this answer by Christian Feuersänger to the same question reference above, but suffice it to say that I do not fully understand it and therefore struggle with adapting it to my case.)
documentclass[tikz]{standalone}
usepackage{calc}
usepackage{ifthen}
begin{document}
    begin{tikzpicture}[scale=1]
        foreach y [count=n] in {  
            {0,100},
            {150,200},
        } 
    {foreach x [count=m] in y {
        fill[ifnumx<100 {white!x!red} else {red!{x/2}!black}fi] (1*n,-1*m) rectangle ++(1,1);
    }}
    end{tikzpicture}
end{document}

Can someone tell me, what my mistake is or maybe even how to solve this more efficiently?

koleygr · Accepted Answer

Not sure if you want to achieve this exact result, but the code with corrections is this one:
documentclass[tikz]{standalone}
usetikzlibrary{calc}
usepackage{xcolor}
usepackage{ifthen}
begin{document}
    begin{tikzpicture}[scale=1]
        foreach y [count=n] in {  
            {0,100},{150,200}}
    {foreach x [count=m] in y {
    pgfmathtruncatemacromyx{x/2}%
    xdefmyColor{ifnumx<100 white!x!redelse red!myx!blackfi}%
        fill[color={myColor}] (1*n,-1*m) rectangle ++(1,1);
    }}
    end{tikzpicture}
end{document}

Edit:
In order to not lose the range (0--400) you can add some green in the red color (half green.. to make it yellowish) and keep the RGB definition (0--255)... So, your range will be 2times 255 = [0--510]):
documentclass[tikz]{standalone}
usetikzlibrary{calc}
usepackage{xcolor}
usepackage{ifthen}
begin{document}
    begin{tikzpicture}[scale=1]
        foreach y [count=n] in {  
            {0,100, 200},{150,200,400}} 
    {foreach x [count=m] in y {
    pgfmathtruncatemacromyx{ifnumx>255{x/2}else{x}fi}%
    pgfmathtruncatemacromyy{ifnumx>255 127else 0fi}%
    definecolor{myColorm}{RGB}{myx,myy,0}%
        fill[color={myColorm}] (1*n,-1*m) rectangle ++(1,1);
    }}
    end{tikzpicture}
end{document}

Edit2:
For more "gradient" approach use the functions:
pgfmathtruncatemacromyx{ifnumx>255{255}else{x}fi}%
pgfmathtruncatemacromyy{ifnumx>255{x-255}else 0fi}%

Using numbers higher than 100 in tikz color definition, when plotting from csv

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