5
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I have the following problem. I'd like to add a legend to MatrixPlot. Each colour should have a legend entry. I used PlotLegends, which in principle works. However, if I use more than five colours, this doesn't work anymore.

a = RandomInteger[{1, 6}, {50}];
MatrixPlot[{a}, ColorRules ->
                 {1 -> Red, 2 -> Blue, 3 -> Green, 4 -> Gray,5->Yellow,6->Orange}]
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6
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When Automatic fails the same thing can, luckily, be done manually:

rules = {1 -> Red, 2 -> Blue, 3 -> Green, 4 -> Gray, 5 -> Yellow, 6 -> Orange};

a = RandomInteger[{1, 6}, {50}];
MatrixPlot[
 {a},
 ColorRules -> rules,
 PlotLegends -> SwatchLegend[rules[[All, 2]], rules[[All, 1]]]
 ]

Example legends

A horizontal version (this is an update in response to Bob's new answer) can be achieved thus:

PlotLegends -> Placed[
  SwatchLegend[rules[[All, 2]], rules[[All, 1]]],
  Bottom
  ]

Horizontal legends

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a = RandomInteger[{1, 6}, {50}];

colors = {1 -> Red, 2 -> Blue, 3 -> Green, 4 -> Gray, 5 -> Yellow, 
   6 -> Orange};

Column[{
  MatrixPlot[{a},
   ColorRules -> colors,
   ImageSize -> 400],
  SwatchLegend[
   colors[[All, 2]], 
   colors[[All, 1]],
   LegendLayout -> "Row"]},
 Alignment -> Center]

enter image description here

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