12 watts is more that enough for leafy greens. You can actually get away with 1 watt. There was someone on reddit who experimented with 1 watt LED with a single pepper plant and had no issues. Even got it to flower. Majority of Aerogarden's LED lights are 20 watt full spectrum.

I'm using these on a rack shelf: Grow light

I have 2 placed on each rack. The rack is 3ft long.

They pull between 25-30 watts per panel. You get two panels. Pretty sweet deal for low budget.

Color does matter. Blue light, for example helps encourage vegetative leaf growth. Red light, when combined with blue, allows plants to flower. Red is the least important one for leafy greens but still needed. Depending on how much light of each is given can change the plants growth behavior.

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How many plants are you trying to grow? Your light requirements can change based on your space.