How could I make visible the convex hull of 3 point inside this region?

Question

Small version of the question How can I control the color and the Thickness of this graph??

a1 = 0.2;
a2 = 0.5;
a3 = 0.6;

p1 = {1, 1, 1};
p2 = {2, 2, 2};
p3 = {1, 2, 3};
chull = ConvexHullMesh[{p1, p2, p3}];
Show[HighlightMesh[chull, Labeled[1, "Index"]]]

---

The question complete

How can I control dthe color and the transparency of the triangle of 3 points in this code?

Manipulate[
 segmento[a_] := {2, 5, 4} + a {3, -2, -1};
 curva[a_] := {2 + 3 a, 5 - 2 a, 4 + 2 a - 3 a^2};
 p1 = curva[a1];
 p2 = curva[a2];
 p3 = curva[a3];
 chull = ConvexHullMesh[{p1, p2, p3}];

 pp3 = ParametricPlot3D[
   segmento[a] + h*(curva[a] - segmento[a]), {a, 0, 1}, {h, 0, 1}, 
   PlotRange -> {{0, 5}, {0, 5}, {0, 5}}, 
   PlotStyle -> Directive[Green, Opacity[0.75`], Specularity[White, 20]]];

 pp2 = ParametricPlot3D[curva[a], {a, 0, 1}, 
   PlotRange -> {{0, 5}, {0, 5}, {0, 5}}, 
   PlotStyle -> Directive[Red, Opacity[0.85`], Specularity[White, 20]]];

 pp1 = ParametricPlot3D[segmento[a], {a, 0, 1}, 
   PlotRange -> {{0, 5}, {0, 5}, {0, 5}}, 
   PlotStyle ->  Directive[Blue, Opacity[0.85`], Specularity[White, 20]]];

 Show[pp1, pp2, pp3, 
  Graphics3D[{Red, Sphere[curva[accionpura], 0.1]}], 
  Graphics3D[{Blue, Sphere[segmento[alfa], 0.1]}], 
  HighlightMesh[chull, Labeled[1, "Index"]], 
  ViewPoint -> {4, 1, 0.75}],
  {a1, 0, 1}, {a2, 0, 1}, {a3, 0, 1}]

to obtain any similar to this

Answer 1

Ignoring the nonworking parts of the code, we can focus on the question

How can I control dthe color and the transparency of the triangle of 3 points?

The answer is to give Style arguments to the HighlightMesh function you are already using.

p1 = {1, 1, 1};
p2 = {2, 2, 2};
p3 = {1, 2, 3};
chull = ConvexHullMesh[{p1, p2, p3}];

Manipulate[
  HighlightMesh[
   chull, {Labeled[1, "Index"], Style[1, Opacity[n], Red], 
    Style[2, Opacity[n], Red]}]
  , {{n, 0, "Opacity"}, 0, 1}]