Not a Problem
(See screenshot) Tangent g has same color of the other coloured tangents but does not display the same colour.
The same problem 
(See screenshot) Tangent g has same color of the other coloured tangents but does not display the same colour.
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In addition to frndmrsl:
All lines have one colour and additionally one thikness and one opacity (in object properties/style). If a line is above another line, the lower line shines through (dependig opacity). This changes the over all color
Solution: Set the black straight lines (f,h,j) to NOT "Show Object" (hidden)
(click on the blue dot at left in the algebra view or use properties/basic: show object)
It will be easier if you post your ggb file instead of screenshots
I see same colour
Here is the .ggb file...
Les droites g , i et k sont doublées par les droites f , h et j qui sont de couleur noire .
Ok, I finally understood what was going on and I am really sorry because I thought it was a bug. I personally want to thank the users frndmrsl and rami for their patience.
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