Generally, declaring "Light" won't let you target your opponent's monsters.
* Most effects that declare a specific Attribute are focused on summoning/special summoning monsters of that Attribute. They don't generally affect your opponent's monsters.
* "Declare Light" isn't a specific card effect, so we need to know the context.
Example Scenarios:
* "Special Summon a LIGHT monster from your hand": This effect only affects your own monsters.
* "Destroy all monsters on the field that are not LIGHT": This effect targets all monsters that aren't LIGHT, regardless of control.
* "You can Special Summon a monster from your hand that shares an Attribute with a monster on the field": This effect lets you target your opponent's monsters if they have a LIGHT monster on the field, but you must also have a LIGHT monster in your hand.
Important Notes:
* Tualatin's effect is irrelevant to this question. Tualatin's ability to special summon depends on what cards you control, not what attribute you declare.
* Always check the card text carefully. Some effects have unique wording that can affect opponent's monsters.
To get a definitive answer, provide the specific card text that mentions "declare Light."