You just are not putting enough pigment down.. - Think how long a cartridge lasts? Most have less-then 15ml of 'ink' in them. That is three teaspoons. Most are also dye-based, rather than pigment-based, so not a 'solid' colour.
You can get better results, with a white background to apply the decals to, or a suitable colour for your lighter lettering. A bit of a pain, to mask-up though.
As others have said:
The paper / adhesive properties may not like UV.
The inks may not be colour-fast, water-resistant, or UV stable.
You may well not get a 'clean' edge if you have blocks of colour butting right against each other.
A colour-laser may-well give a more robust (in a colour sense) result, but there gamut tends to be less 'photo-realistic'.