There is an almost endless supply of products you can use to color your resin, into any color of the rainbow, any color you can imagine, and all shades of glittering metallics. General rules for coloring your resin:

• Test first! Make yourself a swatch sheet by mixing up small batches of resin with your chosen colorants. Some can cause the resin to 'seize' and harden quicker than expected. Some can cause the resin to not cure hard and remain sticky. Some cause cloudiness, and some can cure into a completely different color to what you expect!
• No more than 10% of your colorant added to the resin by volume. Usually much less is needed than the 10%.
• Consider colorfastness. You want your artwork to stand the test of time.

Types of resin colorants you might want to try

Metallic powders add incredible shimmer and shine as well as a metallic look to your resin. These are my favorites in both silver and gold. You can get different looks depending on whether you simply stir and pour the resin, or if you manipulate it once it's poured onto the surface. I like the natural and organic designs these create, that look perfect for your resin geode pieces. Other pigment powders and mica. As with all resin colorants you should test before use but this is especially true of micas and pigments that aren't necessarily intended for use with resin. Sometimes the mica will form little clumps and seem to be impossible to stir into the resin smoothly, or they may make the resin cloudy, or simply may cure into a completely different color to that you were expecting. However there is a rainbow of colors and shimmering effects on offer with lots of sparkles, so they should be on your must-try list. Inks - India ink, acrylic ink, and alcohol ink.  Pigment-based acrylic inks are much more opaque and light-fast than dye-based alcohol inks. You can get really interesting effects with the alcohol inks in resin but unless your resin has good UV protection, alcohol ink colors can fade over time.  Alcohol is a solvent and is used for resin clean up so use alcohol inks sparingly with resin. Acrylic inks on the other hand have vibrant colors and are really the thinnest form of acrylic paint. They should perform well in resin. India inks are reported to work beautifully too, although I've not tried any of these personally, Acrylic Paint. A favorite of mine simply because it's easy. So many colors, every color you can imagine. Beautiful metallics with sparkle and shine, transparent colors, opaque colors, pastels, rich deep velvety colors, lush full colors of nature - I could go on. Usually, a couple of drops will be enough so it's really economical to use as well. Grab out your favorite paint colors and test them in some resin samples to make a swatch sheet. You'll love the metallics I'm sure. One thing to note about the color shift paints. Those are perhaps my absolute favorites for acrylic pouring, but although the color is great in resin, some of them seem to lose their ability to color shift. Give them a try for yourself and see what you think. The black-gold color shift paint is a winner though - that one looks stunning in resin. Liquid or gel colorants. Great for opaque colors. I like to use the Alumilite white to get a really bright opaque white, although there are several resin tints on the market, and each of the major craft resin brands will produce its own range. The Media Fluid Acrylics also do well. Again, in the highly pigmented form of acrylic paint, I especially like to use black. Just a couple of drops is enough for a very dark and opaque finish. Nail varnish.  Got a fantastic nail varnish color you would love to add to your resin? Most likely it will do beautifully, although you may need to add quite a few drops. Avoid using white, very light colors or the clear nail varnish that has glitter, as it will most likely cause your resin to go bright yellow, either right away or over time. Again, testing is key, as some have reported that it can cause the resin to not cure properly. 

What doesn't work?

I'm very disappointed to report that all of the glitter acrylic paints I've tried really haven't worked well. Both the Martha Stewart which is my absolute favorite acrylic glitter paint, and the DecoArt Craft Twinkles both turned the resin completely cloudy. The Martha Stewart did eventually fade away over time, just leaving the glitter. The DecoArt one is still very cloudy for now, although I've not left this as long to test it. But I suggest if you want to add glitter, that you simply add the loose glitter and don't try the glitter paints.  You might also try painting with the glitter paint and then covering with a clear resin coat - but test first!
To summarise - pretty much give it a try if you think it might work Just remember to do a test first before adding a product you've not used before into your resin geodes and spoiling the whole piece.  In the next section, we'll look at the glitters and other embellishments you can add. The glitters can be stirred into the resin or sprinkled on top - either way, they look fantastic, but I've not added them in this section because they don't dissolve to actually color the resin itself.