Multiple sequence diagrams with smartdiagram in tikz and explanatory text

Question

I want to draw multiple sequence diagrams with smartdiagram in tikz and append some explanatory text.

Here is my minimum working example code:

\documentclass{article}

\usepackage[a4paper, landscape, margin=2in]{geometry}
\usepackage[rgb]{xcolor}
\usepackage{tikz}
\usepackage{smartdiagram}
\usesmartdiagramlibrary{additions}

\definecolorseries{colours}{hsb}{grad}[hsb]{.575,1,1}{.987,-.234,0}
\resetcolorseries[12]{colours}

\begin{document}

\centering
\smartdiagramset{
    set color list={red!10, red!30,red!50},
    sequence item border color=black,
    sequence item text color=black,
    sequence item border size=1.2\pgflinewidth,
    sequence item font size=\scriptsize\sffamily,
    additions={
        additional item shape = rectangle,
        additional item fill color = colours!!+!20,
        additional item border color = colours!!,
        additional arrow line width = 2pt,
        additional arrow tip = to,
        additional arrow color = black,
        additional item font = \scriptsize\sffamily,
      }
}

\smartdiagramadd[sequence diagram]{A1, B1, C1, D1, E1, F1, G1, H1}
{
above of sequence-item1/Step 1,
above of sequence-item2/Step 2,
above of sequence-item3/Step 3,
above of sequence-item4/Step 4,
above of sequence-item5/Step 5,
above of sequence-item6/Step 6,
above of sequence-item7/Step 7,
above of sequence-item8/Step 8
}


\smartdiagramconnect{to-}{sequence-item1/additional-module1}
\smartdiagramconnect{to-}{sequence-item2/additional-module2}
\smartdiagramconnect{to-}{sequence-item3/additional-module3}
\smartdiagramconnect{to-}{sequence-item4/additional-module4}
\smartdiagramconnect{to-}{sequence-item5/additional-module5}
\smartdiagramconnect{to-}{sequence-item6/additional-module6}
\smartdiagramconnect{to-}{sequence-item7/additional-module7}
\smartdiagramconnect{to-}{sequence-item8/additional-module8}

\smartdiagram[sequence diagram]{A2, B2, C2, D2, E2, F2, G2, H2}

\smartdiagram[sequence diagram]{A3, B3, C3, D3, E3, F3, G3, H3}

\begin{tikzpicture}[overlay]
    \node[left = of module1, xshift = -90mm, yshift = 45mm] {C 1};
    \node[left = of module1, xshift = -90mm, yshift = 27mm] {C 2};
    \node[left = of module1, xshift = -90mm, yshift = 12mm] {C 3};
    
\end{tikzpicture}

\end{document}

Questions

1. Space between first & second sequence diagrams and space between second & third sequence diagrams are not same.
2. Increase the distance between first sequence diagram and explanatory text in boxes above it.
3. Put text like C 1, C 2 and C 3 in boxes.
4. How to change color of a single and multiple items for a specific sequence diagram only.

Answer 1

1. Space between first & second sequence diagrams and space between second & third sequence diagrams are not same.

Empty lines in latex denote a new paragraph. The more empty lines you add between your smartdiagrams, the bigger the space is going to be.

2. Increase the distance between first sequence diagram and explanatory text in boxes above it.

You can change the spacing with additional item offset

3. Put text like C 1, C 2 and C 3 in boxes.

You already put boxes above the diagram. You can use the same technique to put boxes in front of the diagram.

4. How to change color of a single and multiple items for a specific sequence diagram only.

You are hard coding the list of colours in set color list. If you want to change one or more colours change them there.

\documentclass{article}

\usepackage[a4paper, landscape, margin=2in]{geometry}
\usepackage[rgb]{xcolor}
\usepackage{tikz}
\usepackage{smartdiagram}
\usesmartdiagramlibrary{additions}

\definecolorseries{colours}{hsb}{grad}[hsb]{.575,1,1}{.987,-.234,0}
\resetcolorseries[12]{colours}

\begin{document}

\centering
\smartdiagramset{
    set color list={red!10, red!30,red!50},
    sequence item border color=black,
    sequence item text color=black,
    sequence item border size=1.2\pgflinewidth,
    sequence item font size=\scriptsize\sffamily,
    additions={
        additional item shape = rectangle,
        additional item fill color = colours!!+!20,
        additional item border color = colours!!,
        additional arrow line width = 2pt,
        additional arrow tip = to,
        additional arrow color = black,
        additional item font = \scriptsize\sffamily,
        additional item offset = 1cm,
      }
}

\smartdiagramset{set color list={black!50, red!30,red!50}}
\smartdiagramadd[sequence diagram]{A1, B1, C1, D1, E1, F1, G1, H1}
{
above of sequence-item1/Step 1,
above of sequence-item2/Step 2,
above of sequence-item3/Step 3,
above of sequence-item4/Step 4,
above of sequence-item5/Step 5,
above of sequence-item6/Step 6,
above of sequence-item7/Step 7,
above of sequence-item8/Step 8,
left of sequence-item1/C1
}
\smartdiagramconnect{to-}{sequence-item1/additional-module1}
\smartdiagramconnect{to-}{sequence-item2/additional-module2}
\smartdiagramconnect{to-}{sequence-item3/additional-module3}
\smartdiagramconnect{to-}{sequence-item4/additional-module4}
\smartdiagramconnect{to-}{sequence-item5/additional-module5}
\smartdiagramconnect{to-}{sequence-item6/additional-module6}
\smartdiagramconnect{to-}{sequence-item7/additional-module7}
\smartdiagramconnect{to-}{sequence-item8/additional-module8}
\smartdiagramset{set color list={red!20, red!30,red!50,green!40,orange!40,black!50,black!50,black!50}}
\smartdiagramadd[sequence diagram]{A2, B2, C2, D2, E2, F2, G2, H2}{left of sequence-item1/C2}
\smartdiagramset{set color list={red!20, red!30,red!50,green!40,orange!40,yellow!40,magenta!40,brown!40}}
\smartdiagramadd[sequence diagram]{A3, B3, C3, D3, E3, F3, G3, H3}{left of sequence-item1/C3}


\end{document}

Answer 2

\documentclass[a4paper,12pt]{article}
\usepackage{smartdiagram}
\usepackage{geometry}


\begin{document}
\begin{center}
\smartdiagram[sequence diagram]{Adgg,Bdgg,Cdgdg,Dgdg,Effdf,Fhgh}
\end{center}
\end{document}