Paint artwork on pressure treated lumber
There are aspects of the question that relate to paint, the material; painting, as in applying a coating; and painting, as in creating decoration.  I'll try to differentiate these.  In terms of paint, the material, there's a basic difference between a paint and a stain.  I'll use "paint" to refer to a material that coats the surface and dries into a "shell" of material, and "stain" to refer to a colorant that is absorbed into the wood.  That's an over-simplification, but close enough for the purposes of this discussion.

Dealing with pressure treated lumber

Pressure treated lumber stays wet for a long time.  The chemical treatment is forced deep into the wood and it can take over a year for the wood to thoroughly dry out.  

Applying a paint-like material to pressure treated lumber isn't recommended, at least during the first year or two.  Covered areas prevent the wood from drying evenly (causing warping and worse splitting).  The more area that is covered, the worse the problem.  So a few tiny decorations would be less bad than applying a base coat to the entire piece.  

Paint-like material doesn't adhere well until the wood is dry.  When the surface is dry but moisture is still high internally, you get blisters (domes) in the paint, and other symptoms.  

The chemical treatment saturates the wood.  While the wood still feels wet, there isn't much of anything that will stick to it or get absorbed.  Even when the surface feels dry, while there is still high internal moisture, finishes won't go on evenly.  This is especially a problem for stains, and the larger the area, the more noticeable it will be.  

Once the wood is dry to a reasonable depth, stain is typically the best type of finish, whether you're talking about covering all or most of the piece, or applying decorations that will be big enough to be visible from a distance.  It is absorbed into the wood instead of forming a surface coat.  There's a broad range of stain colors, and you could do some artwork with it.  

Decorating the lumber

In terms of decorating the wood, if you want the decorations visible, you will need to contend with what will be the wood's color as the background.  Once the wood dries and weathers a little, it will be a medium gray when dry and can get dark when wet (and water will absorb into the wood, so it will remain dark for awhile until it dries out again).  

Paint creates a solid surface color, so the visibility of artwork created with it won't be affected by the color of the wood.  However, you would want to wait at least a year before using paint for decorations.  Then you will have a problem of paint having a limited life on pressure treated lumber.  Pressure treated lumber doesn't age well when exposed to the elements, and the wood surface deteriorates under the paint, so the paint shell deteriorates.

Stain is much better suited for pressure treated lumber.  However, plain stain modifies the color of the wood more than creating a specific color.  So just applying stain in a pattern to create artwork won't be nearly as visible as paint.  There are a couple of things you can do to improve the situation.

One is to first apply a background coat of a very light color.  You can either apply it to the entire piece as a protective sealer, or just as a background in the area of each decoration.  Bear in mind that stain works by being absorbed into the wood, and most stains you'll find for outdoor use also seal the wood.  The heavier you apply the base color, the more effective it will be at making the background a light color, but it will also seal the wood more, so the colored stains you apply on top for decoration won't absorb as well.  It's a tradeoff.

One other consideration, plain stain is a very thin liquid, which will be runny, especially working on a vertical surface if you decorate after the post in erected.  It will be even worse if you seal the surface with a base coat.  So you will need to control the amount you apply.  You will have a mess if you try to apply it heavily.

A better solution is to use pigmented stains, often referred to as solid color.  Pigmented stains are a mix of stain and solid particles of color.  The liquid gets absorbed into the wood and the pigment stays at the surface, making the color more intense and opaque.  Pigmented stains are closer in consistency to paint, so easier to control when you apply them.  You wouldn't need a color base coat for pigmented stain, just apply it to create the artwork.

Another potential solution (I haven't personally tried this): use a light color pigmented stain as a base coat, perhaps on the whole piece as a protective sealant.  When it is completely dry, paint the decorations on it with oil base paint.  That might work better in theory than in practice; I have no idea.  You could try it on a scrap piece.