Yes it is difficult to get a good finish but possible.
I have never found any tester pots in tile paint in DIY stores, but you can buy cermic paints in Arts and crafts stores. Hobbycraft stock some which have a good range of colours and mix well. (�3-�5) These are more suited to painting designs rather then achiving a smooth block finish. They are good for stenciling or free hand design and also can be bought in pen form.
If you want to have a pattern of plain coloured tiles you have a few options open to you.
Use one colour in something like a checkered pattern. By either using paint or replacing the tiles.
It is easy to replace tiles, but you must ensure that the new tiles are the same size. Some metric/ imperial equivelants are just a mm or 2 off !!
If you go with the tile paint (�11-�15a tin) to get the best chance of a good finish.
1. Clean the tile really well. leaving no traces of grease soap or fingerprints.
2. Mask off using masking tape to the edge of the tile mask the grout as well.
3. Most people find a small gloss roller gets the best finish. Although if you prefer a brush use a varnish brush.
4. Practice on some spare tiles first and start where small errors wont be noticed!
5. As undercovers found layers of paint dont help with tilepaint. If its a spirit based one they just pull off the lower layers.
Light colours are an easier colour change and dont show up flaws as much as dark colours.
Always read the tin first some paints need primers or/and other prep. Some are water some spirit based. Some craft ones need baking to be truly durable b ut will be wipable(not scrubbable)
Anyway hope this helps have fun!