Kraszanki - uniformly colored eggs
The simplest technique of painting eggs (eggs in Polish are called jaja or jajka) which I remember from my childchood in Silesia - was painting the eggs in natural dyes. For instance eggs cooked in onion skins give a pretty medium light brown color or in redbeets give a deep red. Such eggs are still healthy to eat even if the onions penetrated through the eggs changing its coloration inside. The one-colored eggs looks beautiful in the Easter basket if we have each egg in a different color. The less common natural dyes used for coloring eggs were - leaves of birch or alder which give a beautiful yellow color, carrot and pumpkin for orange, fruit of blackthorn for blue or different grasses and nettle for green. Mixing different main colors gives practically all possible color shades. Check the recipe how to prepare kraszanki from onion skins.
Skrobanki - one-colored eggs with scratched patterns by use of sharp tool or pin
The one-colored eggs can be further modified - a delicate pattern can be scratched by use of knife, pin or another sharp tool on uniformly painted kraszanki. If these painted or unpainted but scratched eggs were then painted in dyes then the color would naturally adher to the rough scratched surface. Here are the examples of professional and amateur work.
Frankly, I do not like a word skrobanka because it also means abortion procedure, in Polish. All painted eggs (kraszanki, skrobanki, wydmuszki, oklejanki etc) are known under a general term PISANKI (pysanki).
Oklejanki i naklejanki
In Podlasie (eastern Poland) traditionally the pieces of wool or bulrush were sticked to the eggs forming variety of usually round motives. Such eggs are called oklejanki, check amateur oklejanka. In Lowicz (near Warsaw) the colorful, earlier prepared pieces of paper were adhered to the eggs - called naklejanki.
The majority of the Polish painted eggs which you can buy in souvenir stores in Poland are made of wood. But some are still made of hollow eggs called wydmuszki - which are just blown eggs or egg shells. The name come from the word wydmuchac - blow off - since the inside of the egg was blown away through the hole at the top of the bottom of the egg. The example of the wydmuszki is a photograph above from Bartek collection. Check a recipe how to create a blown egg.
These eggs need to be handled with an extra care, since it is very easy to break it. The eggs which are given to the Easter basket are usually the freshly cooked eggs sometimes painted in some natural dyes.
These beautifully decorated eggs are placed in the swieconka on Holy Saturday as a symbol of new life and Christ's Resurrection from the tomb.
I recommend Decorating Eggs: Exquisite Designs with Wax & Dye by Jane Pollak