What color was the area of the model you were applying the decal to?
Light colored printing from an inkjet will not cover well on a dark area. The printer assumes you are printing all colors onto white. Dark colors print fine, because ordinarily you apply dark lettering and designs over a light color on the aircraft.
For very light colors you must print on white decal paper which becomes a pain. The white is everywhere, so you must either trim the decal perfectly, or else print the graphic with a background color that matches the color of the area over which it will be applied. This takes some testing and a knowledge of color theory to do perfect matching. However, the closer you DO trim, the less any imperfections in background color affect the appearance.
I am hoping to send an article on white and light colored homemade decals to FSM soon.