If you want to use coloured lighting in your garden lighting scheme there are two main ways of doing it.
Colour filters are glass discs which either fit loose inside a spotlight or have a clip which fits onto the front of a lamp (usually 50mm diameter) to fit a 12v MR16 or flat fronted 240v GU10 LED lamp used in most garden spotlights. They work either by absorbing the colours of light you don’t want (absorption filters) or reflecting the unwanted colours back into the lamp (dichroic filters), leaving just the desired colour to shine though. Lighting for Gardens offers a choice of pale blue, blue, green, purple, red and yellow colour filters from stock plus the “moonlighting” filter, sometimes called a daylight correction filter, which raises the colour temperature by approximately 2200 degrees kelvin, to turn a warm white lamp into a cool white or a cool white lamp into pale blue light, that slightly cool lighting effect typical of moonlight. Most filters clip onto the front of an MR16 or GU10 lamp, but some spotlights have custom honeycomb filters which fit into the front bezel instead - look at the accessory list for the spotlight you are proposing to use and check that the filter you want to use will fit.
Coloured l.e.d. RGB lighting provides much more vivid, saturated colour than colour filters and gives you the ability to select your choice of colour to suit your seasonal mood, or allow fitting to cycle through pre-set colours.