As you probably noticed, in real world there are many different species of slugs and many of them are of different colors. Why do not use different kinds of giant slugs in our campaigns?
Roll 1d10 to define color of encountered giant slug.
1 - Black. Slug's mucus and blood is highly poisonous. Anyone who touches it must save vs. Poison or lose 3d6 HP.
2 - Brown. Slug's mucus can corrode metal and transform it into a rusty heap within seconds. Any weapon made of iron/steel/etc. that hits slug will be destroyed after dealing damage.
3 - Red. Giant Slug has 50% fire resistance and can breathe fire.
4 - Green. Giant Slug's mucus is highly corrosive. Acid can damage equipment and cause grievous wounds (1d6 damage per touch).
5 - Yellow. Slug can emit clouds of stinking gas (effect may besimilar to the Stinking Cloud spell, if you use it). Its bite is poisonous (save vs. Poison or 1d6 additional damage).
6 - Cyan. Slug has 50% acid resistance. Can breathe acid clouds (not just spit).
7 - White. Slug's mucus is highly sticky. Any weapon that hits the monster has 25% chance to be trapped. Slimy trace left by the slug is very sticky too, so beware your steps!
8 - Purple. Slug has 100% poison resistance. Its bite is lethal - victim must pass save vs. Poison or die immediately.
9 - Grey. Slug's skin is thick and rubbery. Increase its AC by one step and add one HD. Its meat is edible and rather tasty, but stinks horribly.
10 - Golden. Roll two effects!
In addition, there is 10% chance that encountered Giant Slug will have some additional abilities, determined by its secondary color (of course if it is not identical to slug's primary color!). Additional color can form dots, stripes or larger spots on monster's skin. Roll 1d10 to determine slug's secondary abilities:
1 - Green. Slug's acid and poison resistances are increased by 25%
2 - Yellow. Slug's movement is increased by 25%
3 - Black. Slug can regenerate 1d6 HP per round of combat.
4 - Red. Slug's fire resistance is increased by 50%
5 - Cyan. Slug is immune to poison.
6 - Silver. Slug's magic resistance is increased by 25%
7 - Brown. Slug's spit range is increased by 25%
8 - White. Slug's spit damage is increased by 1d6.
9 - Blue. Slug is highly intelligent. 10% that it can talk.
10 - Gold. Slug is immune to magic.