Fix the group colours for consistent colouring.

[jdunkerley]

Pull Request Description

Fix the group colour to a fixed set.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  TypeScript,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• Unit tests have been written where possible.

[radeusgd]

Btw. shouldn't a node containing a vector literal [1, 2, 3, 4] belong to the "Constants" group also? I.e. be colored the same way as Table.new?