One idea is on the principle of very simple dominoes, with the same size dots in different colours used for each end. These are suitable for small children so use large cards about half the size of a postcard. Twelve or eighteen are sufficient.
Square cards can be cut, with a strip of different-coloured gummed paper on each edge mitred at the corners. Make a set of six with colours arranged so that by matching colours the cards can be placed in a straight line. For older children these can be designed so that they will only fit in a line if they start with a particular colour.
A similar idea is pairing cards which can either be square or rectangular. Use coloured Sellotape to make stripes involving two colours. Each card should pair with only one other and six pairs is sufficient. Make sure that two pairs are very easy so that even the least able three-year-old can ‘find the partner’. Or use vinyl wallpaper from pattern books to provide different pairs of colours in the same set of papers mounted on squares of board.
Variations of these can be made more difficult, for example the colour dominoes could have two circles of colour at each end. To provide a different matching feature, such as a design, use wallpaper border instead of plain colour strips for the square game or use patterned vinyl wallpaper with two colour variations for each pattern.