Boxed hair dyes are easy to apply and easy to find. BUT. They are not good for your hair.

Why?

Because box dyes generally use the highest concentration of hydrogen peroxide a.k.a developer.  This is used to remove the pigments of the hair and allow for the hair colour to enter. THIS CAN CAUSE DAMAGE TO HAIR.  The Concentration range from 10vol (minimal effects) to 40vol (bleach).   It is important to choose the correct volume.  For example, this hair dye will tint hair from..

Black > Brown
Light brown > medium Brown
Blonde > Light Brown

This means that this product will work on pretty much every hair colour.  The concentration of hydrogen peroxide must be strong enough to lighten dark hair. This means a person making a minor change will expose their hair to much higher concentrations then necessary.

What would be better?

If you do your research and feel comfortable dying your hair and choosing all your products.  It is better to buy your own colour and developer. If not, go to the salon. This is avoid any unnecessary damage.

I am not a professional, use the advice provided at your own risk.

I have been colouring my hair wild colours for years now.  Its fair to say that I have some experience with hair dyes.  So these are MY OPINIONS about these two dyes.

                                              Manic Panic            vs             Special effects

Bright colours                             Yes                                              Yes
Long lasting                        No (1-2weeks)                            Yes(3-4 weeks)
Texture                                 Thick Paste                                 Thick Liquid      
Range of colours                        Many                                          Many
Price                                            $$                                                  $
Availability                            easy to find                                   harder to get

I've used manic panics ultra violet and electric blue on my hair and my boyfriends' hair.  For both of us, it was bright for a couple days and then began to seriously fade.  I know a few people who swear by manic panic; but it just doesn't work for me.

Special effects works great.  I have used blood red, deep purple and atomic pink.  These are very bright and long lasting hair dyes.  I would certainly recommend special effects to anyone who wants to use a semi-permanent hair dye.