What do the decals say?
If you're advertising a business, go with a darker hue to make the decal stand out. Titanium grey or a metallic to match the wheels and Mazda emblems would be a good start and would be my second choice (first for me would be no decals, since I'm a 40+ manager in a fortune 100 company... when I take someone to lunch it can be serious... but that issue was already handled above).
If you're going for the Sponsor "look", remember that sponsors want their name to pop out at you - a brighter contrast or plainer "look" may be necessary - look at black, navy, even burgandy maybe. OR... use actual sponsor decals (could look a little NASCAR though...)
For a more subtle appearance (though if you're considering decals, subtle may not be the "look" you're going for...) check an orange, white, or shade-on-shade yellow (pastel version of your color would look slick).
Another option with any color would be a shadowed appearance. As an example: get the same decal in both orange and white, apply the white one first then the orange slightly off center to the white, giving a rim of white on the one side of the orange lettering - you can substitute any two colors, I just used orange and white as an example, perhaps a "creamsicle" example...