Metallic or iridescent acrylic paints (and inks) can be used in the same way as any other acrylic paints. While they make an interesting departure from the usual dark brown used for antiquing, metallic paints are chiefly used for mokume gane and crackled-paint effects.
Each person has his or her own favorite brand of paints for crackling or using in mokume gane. Every brand acts a bit differently from the others, so keep looking until you find one you like. Combining different paints can create appealing results, too, particularly when crackling, since each paint will crackle in a slightly different way.
If you have crackling in mind, you'll want to avoid paints intended for fabrics. Fabric paints, such as the Lumiere paints from Jacquard, are formulated to stretch, so they tend not to crackle like other acrylics do. While Lumiere paints provide beautiful results in mokume gane, for crackling, you'll likely do better with the metallic lines from FolkArt or DecoArt (Dazzling Metallics). Some people have found that "crackle-ability" can vary even within one brand of paint. One bottle of one color may crackle beautifully while another does not. Experimentation is key.
Metallic inks can also be used in many of the same ways as metallic acrylic paints. For instance, the Posh Impressions line of metallic inks crackle well and also look very pretty in mokume gane. Daler-Rowney's Pearlescent Liquid Acrylic Colors (which are located in the calligraphy section of most craft stores) are available in a line of beautiful colors that crackle beautifully.
If you have mica powders, it is possible to make your own metallic or iridescent paints from them. Just mix a little powder with a squirt or two of Future Floor Finish (or any other polymer clay-friendly medium of your choice) until you reach the desired consistency. This type of paint works well for crackling.