Do you have to use their shampoo with it

That's what they recommend, I couldn't say how much a difference using the conditioner with or without the same shampoo makes.

will this work on dirty blondes

Yes, it will bright and do a great job. I dye my hair from brown/grey to dirty blonde (a not too light blonde color). What it is especially good for is that you will be able to go much longer between dye jobs. That is a great time and money saver, not to mention making it much easier on your hair. I love this shampoo because it makes my hair much more manageable. Hope this helps. Of course, if you don't dye your hair, it really would brighten your natural dirty blonde color and make it sparkle! Good luck!

Does it work on colored hair (dark brown)? Thanks

I use it that helps lighten my hair. I have light blonde hair it does wonders for me. Not sure about dark hair.

I'm a guy. I keep my hair pretty short, a little spikey, but short, oftentimes almost military short? Do I even need this since my hair will be cut

Yes if you want your hair lighter or just the tips...use this with the whole set shampoo and conditioner but remember after the spray to blast it with heat from the hair dryer or maybe it might work with the heat of the sun like sun -in does. I used to use sun in and it actually worked better in the summer when I would go in the sun or at the beach with it...it would really bleach out my dirty blonde hair and made it look like a nice extra light platinum blonde!