plotting - Legend Matrix Plot

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}]

Answer

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]]]
 ]

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
  ]