The problem with having two groups of 4 smaller fish is that small fish generally tend to be shoaling species which need at least 6 of the same species to feel safe. They live in groups of hundreds if not thousands in the wild and if there aren't enough of them, their instincts tell them something has eaten the rest of the shoal so they'd better be on the look out for the predator or they'll be next. This leads to the fish becoming very stressed, and stressed fish get sick easily.
In 70 litres you should be able to fit 2 shoals of 6 or even 8 fish each, provided you choose species that don't grow more than ~3cm.
If you want brightly coloured fish, look at
ember tetras. They don't look much in the photo in the profile on here, or in the shops, but when they settle in your tank, they are bright red. If you look at the red fish at the top left of the photo
here, these are my ember tetras.
Other bright tetras include neon/cardinal tetras - 2 related species but with small differences.
However, I would not get these fish if you have very hard water as both prefer soft water.
For hard water, look at livebearers - platies come in bright colours, as do the smaller enders, though males only with livebearers or you'll be over run with fry.
Soft water centrepiece fish - the laready mentioned dwarf gourami or honey gouramis. Although the female honey gourami is greyish beige in the natural form, there are two colours developed by selective breeding which are more common, and the females are the same as the males - yellow and red (also known as red robins). There's one of my yellow honeys in that photo.
If you got smallish fish, you could try a group of cherry shrimp, which are bright red. But at least half a dozen of them, not just two. With any shrimp you would need somewhere for them to hide when they moult as they are easy prey for any fish until their new skin hardens. Some sort of cave, and lots of plants, real or synthetic.