I'm well aware of why it looks purple. On this site, those are referred to a "burple" lights. Typically lights bought on eBay or Amazon boasting "1000W output". In reality, they're normally about 60-100W of true output and new growers get discouraged by why their plants don't work out the way they had hoped. Any light in that category is called a "Burple".
That light is seriously under-powered for what you're trying to do here. The light is your problem. At 9-weeks under a good light, your plants should be 4-ft tall and look like shrubs you'd plant outside a house. For reference. These two (bigger) plants are in week 5 of veg (18/6). The smaller one is in week 3. (I just flipped them to 12/12 this week.) Big ones are 3' tall and 2' wide.
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