How to detect a dark color in RGB triplet?

I am plotting a map with many subdivision and colors.

I would like to automatically draw the borders with contrasting color with respect to the facecolor of the subdivision.

I am using the sum of the RGB triplet but I am not having perfect matchs.

Is there another way to do that?

My results until now:

Answer 1

[Image Analyst]

I don't know what you do on the border between light and dark regions. But anyway, when you're drawing the boundary around a region get the mean RGB of that region -- summing the values is fine, or equvalently, taking the mean. Then you can say

meanRGB = mean([regionsRedValue, regionsGreenValue, regionsBlueValue]);
if meanRGB < 128
    % It's dark, so draw with white line.
    lineColor = 'w';
else
    % It's bright, so draw black line.
    lineColor = 'k';
end
plot(x, y, '-', 'Color', lineColor, 'LineWidth', 2);

[David Franco]

For Gama 0-1 should I use 0.5 instead of 128?

[David Franco]

I am already using

if mean(color) < 0.45 % tried with median(color) < 0.45 too
    mapshow(Snew(results(ii,2)),'FaceColor',color,'EdgeColor',[1,1,1]); % white border
else
    mapshow(Snew(results(ii,2)),'FaceColor',color,'EdgeColor',[0,0,0]); % white border
end

But the results aren't good enough... Sometimes light regions get white borders: