Treat underglazes like you would artist's paints. Mix them. If a yellow is too brash, add a little white. If a blue is too dark, add a lighter blue--not white--and you'll find you still have a dark blue, but it won't look black. There's a great vibrant blue (Marine Blue) that should work great to brighten rather than lighten the dark blue.
I mix orange with yellow to achieve the interior of an orange, or mix with red to create a beautiful persimmon. Amaco's colors are so stable, and true to the color on their test tiles, that they can be used in a very painterly way. The only ones that still give me problems are the greens and very pastel blues and grays. If you want a lighter color, white is probably not your best choice. Want to lighten a red? Try adding yellow with a tiny touch of white (unless you want pink) Want to darken a red? try green with a tiny touch of black.
Regardless of whether you paint on bone dry (as I do) or bisque, another firing at cone 06 is wise. I've never had underglazes run if the ware has been fired after application.