Whether you're just learning how to draw or are a seasoned pro, the key thing to consider, no matter what palette you're using, is colour harmony. Do the colours play well together? Are they all competing for the viewer's attention? Another important factor is what kind of mood is are you trying to set for your audience?
While your painting techniques might be spot on, what makes for an effective painting is how all the parts come together as a whole. Colour, value, shape language, composition and rhythm all work together to craft a compelling image. If you're relying on a bombastic colour scheme to save your painting, you may have other issues that need to be addressed.
For this article, I painted over a greyscale drawing several times, because the light sources change in each image. My two favourite Photoshop colour tools for doing this are Color Balance and Levels adjustment layers. In a Levels adjustment, you can tweak the red, green, and blue channels individually. Gradient Overlay is also a wonderful adjustment layer to quickly assign one colour to light values and another colour to dark.
The problems with using bright, saturated colours usually arise when you overuse them. But it really depends on the painting and what you're trying to convey. But if you can pull it off, a 'garish' palette isn't garish at all.
Artist secret: Switch colours quickly
When I'm having trouble selecting a colour palette I sometimes let the computer help me decide. There are several online apps to help you. color.adobe.com and colourlovers.com feature great web interface tools that enable you to construct a colour scheme on the fly.