Example 1A - By name
string color = Color;
Example 1B - By name with spaces
string color = #{Product Color};
Example 2 - By id
string color = customfield_12345;
Example 3 - By alias
string color = productColor;
NOTE: This script assumes that it is running in the context of an issue. Like in a workflow script or listener for example. If you would like to test this script from the SIL Manager, add an issue key to test with under the Run Configuration setting (Run > Run Configuration > Context).