In the code below, I do not understand why the label "Arbre 2" does not have the color you want, that is to say the same as the label "Arbre 1".
Thank you.

\usemodule[tikz]
\usetikzlibrary[calc]

\definecolor[A2][c=0.60, m=0.00, y=0.00, k=0.10]
\definecolor[H1prime][c=0.50, m=0.00, y=1.00, k=0.00]

\starttext
\starttikzpicture[line width=0.3mm, >=stealth, x=1cm, y=1cm,line cap=round, line join=round]
\tikzstyle{epais}=[line width=0.5mm, line cap=butt]
\tikzstyle{tres epais}=[line width=0.8mm, line cap=butt]
\draw [tres epais] (6.02,3.63)-- (8.88,3.64);
\draw (6.02,3.63)-- (6.03,0.77);
\draw [tres epais] (6.03,0.77)-- (6.02,3.63);
\draw [tres epais] (8.88,3.64)-- (8.89,0.78);
\draw [dashed] (-3.38,3.6)-- (8.89,-0.09);
\draw [dashed] (8.89,-0.09)-- (8.89,0.78);
\draw [dashed] (-3.38,3.6)-- (6.02,3.63);
\draw [A2,epais,domain=-1:2] plot(\x,{(-8.9-4.4*\x)/-3});
\draw [A2,epais,domain=1.5:4.5] plot(\x,{(--1.58-5.12*\x)/-4.72});
\draw (8.84,4.4) node {\color[H1prime]{\bf Arbre 1}};
\draw (-2.96,4.4) node {\color[H1prime]{\bf Arbre 2}};
\node at (8.14,0.8)  {\small piquet 4};
\draw [A1](8.84,-0.3) node {\bf Georges};
\node[rotate=45] at (0.28,1.88)  {\color[A2]{\bf Ravin}};
\node[rotate=90] at (9.4,1.74)  {\color[B1prime]{\bf longueur mesurée}};
\draw (2.46,4) node {\color[B1prime]{\bf longueur inconnue à estimer}};
\draw[B1prime,<->] (9.05,3.62)-- (9.05,-0.09);
\draw[B1prime,<->] (-3.38,3.8)-- (8.9,3.8);
\node at (8.84,4.0)  {\small piquet 1};
\node[rotate=30] at (6.22,4)  {\small piquet 2};
\node[rotate=30] at (6.22,0.5)  {\small piquet 3};
\stoptikzpicture
\stoptext