4 guys on one base: These are the tiny Tyranid models, correct? Those are called Ripper Swarms. The model in your picture is a Termagant.

There are many way to go about painting minis, but for almost all of them priming is the first step.

You can use hobby paint or even cheaper hardware store paint. When spray priming don’t just sit there hosing the mini, do quick light passes across the minis from different angles.

With tyranids you’re going to want to immediately get in the habit of painting batches of minis rather than one at a time.

You can choose any color you want to prime, but black is a pretty conventional choice. It is a forgiving color if you don’t fully paint all the crevices and undersides later.

Drybrushing is a great way to quickly get color on. You can use a hobby drybrush or a cheap cosmetics brush. A couple of drybrush passes with your creamer color over black can get you surprisingly good coverage.

That’s just one of many options. You could go another way by putting a wash all over the flesh (I think a red or purple wash would look suitably gross on the models). You could also go for contrast paints, which I’m not a fan of using for all purposes but for the fleshy looks of tyranids may be quite successful.