Explanation of colon operator in “: ${foo=value}”

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The expression ${SOMETHING='value'} sets SOMETHING to value if it isn’t already set. This is a useful operator to have in many situations. However, it also returns the assigned value, so if you simply executed

${SOMETHING='value'}

then your shell would try to invoke the command value. This might or might not do something unwanted; at the least it would throw a message “value: command not found”.

To avoid this you can use the no-op :, which evaluates its argument and then throws it away, rather than executing it. This is documented here.

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