Jaxo, usual house paint isn't going to stick well to plastic. I'm not aware of any custom-blend paint that will, so your key here is going to be your primer. Check out Krylon Fusion spraypaint for your easiest option. It's plastic paint and primer in one step, and if you're in luck, they'll have a color close enough to your custom color. Rustoleum also has a Plastic Primer that, if memory serves me correctly, can be top-coated with any paint (double check, though: it might just be any Rustoleum spraypaint) so you may be able to use the house paint you already have.

There are also good bonding primers that claim to coat hard-to-bond surfaces like plastic: Glidden Gripper; Sherwin Williams Adhesion; Zinsser BIN, Cover Stain or Bullseye 123. Ben Moore Fresh Start may work, too, but it recommends at least 3-4 days to cure on non-porous surfaces. Double check that they say they'll bond to glossy, non-porous surfaces and that they're for exterior use. Your best bet with any of the above methods is to scuff it up a little first (sanding sponges work well) and give the primer a day or two to bond before painting.

I haven't had to paint plastic that was exposed to the elements, so I'm not sure how weather will affect the longevity, but since it's not an item that gets handled regularly, I'd imagine it'll hold up pretty well.

I'll try the Rustomleum primer. I think I saw it in the local Lowes.
I ordered three of those plastic motion sensors and I should get them next week.
I have never painted anything before. What's the best way to cover the sensor or any other parts of the unit that I don't want to get paint and primer on? Tape?
What kind of brush should I use to paint something that size? How do you paint so the paint looks smooth and even without clumps and lines when you're not using a sprayer?

You cover the areas you don't want paint on with whatever you have handy. For blocking large areas, I use blue painter's tape on the edges and wax paper in the center. Painter's tape by itself would work for something that small.

I would use a plastic spray paint such as Krylon on that motion detector. Personally, I wouldn't bother trying to match the house paint exactly. Getting it generally in the color ballpark would be good enough for me.

But, if you feel you must match it exactly, you could apply spray-on primer (Rustoleum) and then use your house paint on it.

Here is a link that might be useful: Primer for plastic

Spot-on advice from all above.

Yes, the plastic spray-primer is the critical "bridge" between hard-to-paint-plastic and the final paint-coats.
ANY type of Latex paint, and any type of spray can go over that primer.

IMHO, I too wouldn't try to match the housepaint exactly.
I'd spray it a coordinating color, say....close to one of your trim colors.

Rust-oleum has many good paints....
The Universal is very good, as is the 2X series.
Check out some of the "Specialty" finishes too; such as the light textures, hammered's, or their American-Accents Specialty series of ORB, Satin Nickel, Pewter, etc.

Faron

Agreed that picking a good enough color of spray paint would be my choice...however, if you want to go ahead and use house paint you already have, you may want to try this Preval sprayer. We sell them at my Ace Hardware, but I haven't tried it yet, so I can't comment on effectiveness.

Here is a link that might be useful: Preval sprayer

I already have extra exterior latex paint since the house was painted last year, so I might as well use it and have an exact match.
I might look at a sprayer, but since it is such a small one-time job, I'm not sure a sprayer will be worth buying. It doesn't look like the sprayer above is meant for house paint.
Would it be better to use a brush or foam type applicator to paint?

I would use a brush. A foam applicator won't fit into some of the areas you need to get paint on.

This is one of the actual objects that need to be painted and the color of the stucco it's sitting on is the color it needs to be painted.
So all I need to get is 123 primer, some paint and a very small brush that can get into the crevices?

One thing I'm concerned about is that the sensor needs to be able to move in it's range of motion up and down and side to side so it can be pointed at a specific area.
Painting it where it's supposed to slide could "glue" it in a fixed position and make it stick.

The can of paint I had from last year separated and looked like it went bad, so I bought a quart of new paint.
The new paint is both paint and primer all in one, so it should be able to go on in one step.
I bought some artist's brushes so I can reach into the crevices.

Read the label a little more closely on that Behr paint. I'm betting it has something about "properly prepared surfaces" (ie: sanded & possibly primed with proper primer). I personally wouldn't trust it to prime itself on plastic. I doubt you'll have issues with the sensor movement if you're careful painting.

The instructions say:

Where to use:
Properly prepared coated or uncoated Exterior surfaces. Ideal for Wood, Vinyl, Aluminum, Fiber Cement Siding, Brick, Masonry, Stucco, Doors, Windows, Trim, Shutters, Fences, Garage Doors, Outdoor Furniture, Railing and Wrought Iron.

Countersink nailheads. Fill and sand smooth.
Remove rust, loose or peeling paint, repair imperfections and sand smooth.
Wash off any dirt, grease and chalk with mild detergent and/or powerwash.
Remove mildew stains with a product such as BEHRî PREMIUM MOLD & MILDEW STAIN REMOVER NO. 62. Rinse and allow to dry.
Prepare new or weathered wood with a product such as BEHR PREMIUM 2-IN-1 WOOD PREP NO. 63. Rinse and allow to dry.
Etch bare metals with a product such as BEHR CONCRETE & MASONRY CLEANER/ETCHER NO. 991. Rinse and allow to dry.
Caulk windows, doors and other openings.
Allow new stucco, plaster and masonry to cure 30 days before applying product.
Use BEHR PREMIUM PLUS ULTRA as a primer for repaired or uncoated surfaces, including woods that contain tannins (two primer coats required for redwood and cedar), and heavily stained areas. Lock in stains with the first primer coat; if necessary, apply a second primer coat of BEHR PREMIUM PLUS ULTRA.

I could try to find something to sand it with before painting.
What type of sandpaper should be used on something like this?

A sanding sponge would be easiest for getting around the curves, and all you really need to do is lightly rough it up.

I would still use the recommended Rustoleoum professional primer first. I'm not a fan of one-step prime/paint. Especially with plastic, you need your absolute best shot on the primer. You only get one chance to get primer right.

I bought shutters made of polystyrene material. Can I use Zinsser 123 primer for this material?