Fix error if Color is indexed with a non-string

[Benjamin-Dobell]

At the moment non-string access on Color e.g.

Color[nil]

will raise an error.

Whilst the above example could reasonably be considered user error, index keys (i.e. numbers) are funnelled through when indexing a Color instance. So if you attempt to index a Color instance outside the range [1, 4], rather than getting nil back as you'd expect for non-existent keys on a table, instead an error is presently being raised.

Reproduction

e.g. Entering the following in TTS' chat:

/execute log(Color()[5])

Leads to:

attempt to index a number value

where as you'd expect to see nil logged.

[Benjamin-Dobell]

Force pushed in some tests.