Well, bright colors usually aren't a matter of the program you use^^ As an exmple: The first picture was done in Photoshop, the second one in SAI As someone already said, bright colors have pretty much to do with contrast^^ So, if you want bright colors, you need dark and low saturated colors to get a big contrast^^ To make the hair look more saturated I would suggest you to use yellow or orange for the highlights and a relatively low saturated reddish brown for the shadows^^ You almost never should use black or white until you know what you're doing^^
I don't think its as much about Sai as it is about your choices of colors. For something to appear bright and vibrant it kind of has to have something to compare against. If you have many similar values against eachother then none will stand out that much.
This what I basically edited with Photoshop [link]
And this is how it was before editing [link]
There's obviously a huge difference in the hair color... how can I get such intensity of colors in SAI?