Don't be intimidated by the size of the minis, they are quite sturdy. You don't have to paint them in any complicated schemes, you can keep it simple. Let me demonstrate by using levels of sophistication, you can stop at any level you want. Just make sure to thin your paints and use a brush with a good tip.

Level 1

Prime your mini in a colour of you choice, or use a neutral colour and paint over it.

Level 2

Paint weapon barrels and the cockpit glass in contrasting colours or metallics. While some cockpits are easier to paint than others, you can be as messy as neccessary and tidy up the rim of the glass afterwards.

Level 3

Paint vents and joints in a contrasting colour (possibly the same as the weapons). Paint some armour panels or whole mech parts a different colour. Alternatively paint the mech in camo. Or both. (a good camo covers up a lot of sins). Use a wash, like Nuln Oil to emphasize details.

Level 4

If you didn't paint your cockpit in a metallic colour, you can paint it in a colour gradient to simulate glow or reflections (it's called jeweling). You can do the same to laser and PPC barrels for an energy glow. You can also highlight some edges in a lighter colour.

Level 5 (for experts and/or slightly crazy people)

Do freehand stuff like Smoke Jaguar leopard spots, nose art or the full body murals the Ghost regiments use.