Clean up code style in generators for field colour blocks

[rachel-fenichel]

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Problem

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Request

Follow-up work from #2162.

Clean up the generator code (which is returned as a string) for blocks in field-colour.

For example, Christopher suggested this change in the Python code for the blend block:

-    functionName + '(' + colour1 + ', ' + colour2 + ', ' + ratio + ')';
+  const code = `${functionName}(${colour1}, ${colour2}, ${ratio})`;

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