The difference is marketing. There are slight differences in the phosphors used by different manufacturers, so I won't say that all blue bulbs are the same, but here are the details:
7100K Several years ago Coralife started labelling their blue bulbs with this meaningless designator. Not to be outdone, Custom Sea Life put the same on their blue bulbs. Later both manufacturers removed it because they came out with 8800K and 10,000K bulbs that were white making the 7100K label on a blue bulb look stupid.
Actinic The definition of Actinic is
the property of radiant energy especially in the visible and ultraviolet spectral regions by which chemical changes are produced. By this definition all light is actinic, but the name got stamped onto blue bulbs because Philips started producing a bulb specifically for the chemical industry where the light is used as a catalyst for some polymerization reaction. The term has nothing to do with aquariums or the life in them.
03 This is a color code from the Philips lighting catalog. Nothing more. Philips labels all their bulbs with a two-digit color code, you can buy an "11", a "03", a "05", etc. The "03" designator happens to be blue. So what's the difference between
Actinic and
Actinic 03? Well, if the latter is made by Philips, then it conforms to their specs for color code 03, if not, neither designator means anything.
Remember, the blue color is only the make the tank more pleasing to your eye. Get what looks good to you and don't be fooled by anyone's marketing.
By the way, before everyone went with Chinese OEMs for their bulbs, the Custom Sea Life and some Coralife PCs were made by Panasonic. These companies labelled the bulbs "Actinic", "7100K", "Ultra-Actinic" or "Actinic 03". But we have a copy of the spec sheet from Panasonic. What high-tech specification does the original manufacturer put on their bulb? They call it
blue.
