Art Decko's suggestion of printing a test strip is the only way to find an accurate colour match.
Inkjet printers use Cyan, Magenta, Yellow and Black ink (CMYK) on paper. Most jpg images are RGB (red, green blue) which is composed of light - not ink - shining from your computer screen. In technical terms ink is reflective (reflects light) and RGB is transmissive (transmits light).
Your RGB colour swatches are like apples while your CMYK printer gives you oranges. No wonder they don't match.
You need to convert your colour swatches to CMYK first, using a program like Photoshop. Then use the eyedropper tool to sample the colour, which will tell you its component amounts of each colour.
Let's say for the sake of argument that the colour sampled is the following:
Cyan: 100
Magenta: 0
Yellow: 100
Black: 20
Now make a Photoshop (or Illustrator) file with several coloured boxes about an inch square. Box #1 will be exactly the colours that you've sampled.
Box #2 might be:
C: 100
Y: 100
K: 30
Box #3 might be:
C: 100
Y: 90
K: 20
etc.
The paper (or substrate) you use will also affect the colour, that's why you need to do this test on the actual decal material.
Ink (espacially inkjet ink which is water-based) soaks into paper. When it does, the colour is less bright, because there's less ink on the surface reflecting light. Photo paper is coated to prevent ink absorption. Colours look brighter. Decal "paper" is also impervious to ink, so again, colours will look brighter.
Another thing to bear in mind is the fact that not every colour can be achieved through a mix of CMYK. Some simply cannot be duplicated, such as metallics, neon or flourescents, and some of the brighter oranges and purples.
Looking at your car colour though I think you should be able to match the colour with CMYK.
If that proves to be too much trouble, then perhaps you should go a different route and order some custom decals with just the lettering. Real (as opposed to homemade) decals use opaque ink, so they can print white or yellow onto a clear substrate without the problem of show-through.
I hope this helps. I've been a graphic artist all my life and colour matching is a major issue in that field.
cheers
Val