The color of a light source entails a complicated relationship of different factors, that are important in determining the right light source for your garden.
The advantage of Metal Halide lamps is that they provide high-quality, crisp white light in a variety of different color temperatures that meet the needs of many different users.

Correlated Color Temperature(CCT):
Color temperature is a measure of the color appearance of light not the actual temperature.
..Light sources that appear bright white to blue-white have a high color temperature (3600K and above) and are described as "cool".
..Sources that appear yellow-white to red-white have a low color temperature (3400K and below) and are described as "warm".
..Daylight is considered to be around 5500K and plants grown under this light do quite well.

Spectral Energy Distribution:
When you look at a light source, you perceive seeing a single color, but you are actually seeing thousands of hues. The combinations of different wavelengths of light make up the color we see. The relative intensity of the various wavelengths are used to determine a light source's Color Rendering Index(CRI).


Color Rendering Index(CRI):
The CRI is an indication of a lamps ability to show individual colors relative to a standard. This value is determined from a comparison of the lamp's spectral distribution compared to a black body at the same color temperature. Light sources, such as metal halide lamps, are rated with a CCT; however, CCT does not provide any information on the quality of the color. For this, a CRI is also necessary. In general, the higher the CRI rating of a lamp, the better the different colors will show.


5K - 7K Kelvin: Strong Blue Light
Promotes bushy growth. Ideal for rapid growth phase of plants.
Greatly enhances all-around plant growth when used with super high output, high pressure sodium or 3K warm metal halide lamps.

4.2K - 4200 Kelvin: Cool white Flourescents: Can be used as a supplemental blue lighting when used with a 3K source.

4K - 4000 Kelvin: Neutral Metal Halide:
Best single source for plant growth, producing shorter, bushier growth than 3700 Kelvin and color rendition. Used in general plant lighting.

3.7K - 3700 Kelvin: Softer Metal Halide(coated): This coated lamp is used in general plant lighting and for more rapid growth than 4000 Kelvin produces.

3K - 3200 Kelvin: Warm Metal Halide:
Highest photosynthetically active radiation (PAR) value of all HID
lighting for all phases of plant growth. PAR watts account for the nutritional value of light and are a direct measure of the light energy available for photosythesis.

2.7K - 2700 Kelvin: High Pressure Sodium Lamps:
Redder color mix, used for propagation, blooming, supplemental
greenhouse lighting.
This Bud's for you...:smokin: