Replacing begin{comment} and end{comment} with macro, shorter spelled

The best option for you here is to avoid the functionality of comment and use your own interface. To do this, you define a long version of ans that captures everything up to sna, after which you can do with it what you want. If you don't want to include anything between ans...sna, you can then just discard it. The example below provides both those options; keep both definitions and uncomment whichever you need:

documentclass{article}

usepackage[dvipsnames]{xcolor}

% Similar to includecomment for ans...sna
longdefans#1sna{{%
  color{Plum}#1ignorespaces%
}}
% Similar to excludecomment for ans...sna
%longdefans#1sna{ignorespaces}

begin{document}

setlengthparindent{0mm}

Hello !
ans
I say hello because I have no better idea !
sna
How are you ?

end{document}

When you use the "gobble" version (second, similar to excludecomment), the output resembles this:

With environments defined via includecomment/excludecomment, the end of an instance of such an environment needs to be detected.

The detection-mechanism "assumes" that the phrase denoting the end of the environment can be derived by prepending to the name of the environment the phrase end{ and appending the phrase }.

The detection-mechanism does not trigger expansion of things for detecting that phrase but "expects" to find it verbatim.

Therefore with
      ans → begin{detail} and sna → end{detail}
you can, e.g., do
      excludecomment{detail} ... ans ... end{detail}
, but you cannot do
      excludecomment{detail} ... ans ... sna   :

documentclass[10pt]{article}
usepackage{comment}

begin{document}

newcommand{ans}{begin{detail}}
newcommand{sna}{end{detail}}

excludecomment{detail}
%includecomment{detail}


Hello !
ans
I say hello because I have no better idea !
end{detail}
How are you ?

end{document}

By the way:

      excludecomment{detail} ... begin{detail} ... end⎵{detail} (space between end and {detail}) doesn't work either. It is not the same as end{detail} (without space).

---

If you really absolutely want ans ..sna to work like begin{detail}..end{detail}, you can define a command ans which saves the current color and hacks the commands Enddetailtest and AfterdetailComment (which are components of the detail-environment) in order to have the detail-environment scan for the phrase sna instead of the phrase end{detail}. You need to be picky about the catcode-régime under which things get tokenized—therefore this is tricky.

You can do neither begin{detail}.. sna nor ans..end{detail}.

With ans..sna the same rules apply as with begin{detail}..end{detail}. E.g., sna must be on a line of its own, not preceeded by spaces, ...

documentclass[10pt]{article}
usepackage[dvipsnames]{xcolor}
usepackage{comment}

setlengthparindent{0mm}

begingroup
makeatletter
catcode`^^M=12relax%%%
@firstofone{%
  endgroup%%%
  newcommand{ans}{%
    letsavedEnddetailTest=EnddetailTest%%%
    letsavedAfterdetailComment=AfterdetailComment%%%
    colorlet{savedcurrentcolor}{.}%
    {%
     escapechar=-1relax%%%
     expandafter%%%
    }expandafterdefexpandafterEnddetailTestexpandafter{stringsna}%
    toks@expandafter{%
      AfterdetailComment%%%
      letEnddetailTest=savedEnddetailTest%%%
      letAfterdetailComment=savedAfterdetailComment%%%
      color{savedcurrentcolor}%
    }%
    edefAfterdetailComment{thetoks@}%
    begin{detail}^^Mcolor{Plum}%
  }%
}%

begin{document}

includecomment{detail}

Hello !
ans
I say hello because I have no better idea !
sna

Hello !
begin{detail}
I say hello because I have no better idea !
end{detail}

Hello !
ans
I say hello because I have no better idea !
sna

Hello !
begin{detail}
I say hello because I have no better idea !
end{detail}

bigskip

hrule

bigskip

excludecomment{detail}

Hello !
ans
I say hello because I have no better idea !
sna

Hello !
begin{detail}
I say hello because I have no better idea !
end{detail}

Hello !
ans
I say hello because I have no better idea !
sna

Hello !
begin{detail}
I say hello because I have no better idea !
end{detail}

end{document}

---

With the example above with the ans-command the catcode of the return-character (^^M in TeX's ^^-notation) is changed to 12 in order to have the color-specification processed as if occurring on a new line.

Therefore with the ans..sna-command things are in the color Plum while with begin{detail}..end{detail} things are in the color black.

If you wish the color Plum both with ans..sna and with begin{detail}..end{detail}, you can omit this catcode-trickery and define the detail-environment in terms of specialcomment instead of includecomment:

documentclass[10pt]{article}
usepackage[dvipsnames]{xcolor}
usepackage{comment}

setlengthparindent{0mm}

newcommand{ans}{%
  letsavedEnddetailTest=EnddetailTest
  letsavedAfterdetailComment=AfterdetailComment
  {%
   escapechar=-1relax
   expandafter
  }expandafterdefexpandafterEnddetailTestexpandafter{stringsna}%
  csname toks@endcsnameexpandafter{%
    AfterdetailComment
    letEnddetailTest=savedEnddetailTest
    letAfterdetailComment=savedAfterdetailComment
  }%
  edefAfterdetailComment{thecsname toks@endcsname}%
  begin{detail}%
}%

begin{document}

specialcomment{detail}%
               {colorlet{savedcurrentcolor}{.}color{Plum}}%
               {color{savedcurrentcolor}}

Hello !
ans
I say hello because I have no better idea !
sna

Hello !
begin{detail}
I say hello because I have no better idea !
end{detail}

Hello !
ans
I say hello because I have no better idea !
sna

Hello !
begin{detail}
I say hello because I have no better idea !
end{detail}

bigskip

hrule

bigskip

excludecomment{detail}

Hello !
ans
I say hello because I have no better idea !
sna

Hello !
begin{detail}
I say hello because I have no better idea !
end{detail}

Hello !
ans
I say hello because I have no better idea !
sna

Hello !
begin{detail}
I say hello because I have no better idea !
end{detail}

end{document}

---

To the question:

Is there a way to define sna so that it does the job ?

With the previous two examples the command sna isn't defined at all. ;-)

---

---

When writing this answer and compiling/testing the code, according to the .log-file the following files/releases/versions were in use:

This is pdfTeX, Version 3.14159265-2.6-1.40.21 (TeX Live 2020) (preloaded format=pdflatex 2021.1.17)  29 JAN 2021 23:13
LaTeX2e  patch level 4
L3 programming layer <2021-01-09> xparse <2020-03-03>
[...]
Document Class: article 2020/04/10 v1.4m Standard LaTeX document class
[...]
Package: xcolor 2016/05/11 v2.12 LaTeX color extensions (UK)
[...]
File: dvipsnam.def 2016/06/17 v3.0m Driver-dependent file (DPC,SPQR)
[...]
File: l3backend-pdftex.def 2021-01-09 L3 backend support: PDF output (pdfTeX)
[...]
/usr/local/texlive/2020/texmf-dist/tex/latex/comment/comment.sty

The .log-file only revealed the location of the file comment.sty, not the version.

Looking at the file at the denoted location revealed:

% Comment.sty   version 3.8, July 2016
% copyright 1998-2016 Victor Eijkhout