So it looks in Photoshop
1 only one layer filled with the color of the decal
2) in the RGB channels layer is added in alpha (the numerals are pasted there)
3) It is important that this alpha be with absolute black background and absolute white text (lighter or darker tones will result in a translucent sticker ... which can also be an option according to your intention)
4 So it looks with the RGB and alpha activated, in pink all the transparent area
A strange fact, product of my experience ... each 3d mesh can accept up to 8 stickers, ask who would use both. Well, when you discover that labels are not only useful for creating serial numbers, for example, they paint. Meshes that have no texture, or locate texts that can not be read by low resolution, you may need it.
I hope it is useful, greetings!