To clarify what Chad said, the copper colours are the blueish greeny colours - Copper Green, Petroleum Green, Turquoise, Sky Blue, [Edit: and grey...] in that order (all opaque). If you overheat copper colours you can actually draw the copper to the surface in reddish spots! (This usually just looks dirty, though). Mosaic Green, Rubino Pink and Evil Devitrifying Purple (254) may or may not have copper (I don't really know) but they play well with the copper family.
Any of the copper colours will react with the Sulfur family - Dark ivory, light ivory, red, yellow, coral, orange, brown, etc. You'll always get a black outline where a sulfur family touches a copper family opaque. If you overheat the combination you lose the crispness and get a fuzzy brownish halo. Some combos can hardly handle heat at all - ivory doesn't play well with mosaic green, rubino or EDP. In my opinion!
You can think of the colours that aren't in these families as the non-allied colours. They'll mix safely (no brown blur) with anything! In my experience, white, black, periwinkle, pea green, nile green, opaque cobalt/lapis blue, opal yellow, violet, and (
grey) are non-allied.
[Edit - in recent experiements, I've found Grey to actually be in the Copper family!)
A lot of colours "concentrate" by forming a darker/more transparent centre stripe when they meet a certain other colour, as Lisa/Stephanibeads describes. It's pretty!
Other reactive colours include:
EDP (254 opaque purple) : Spreads into opal yellow with a lovely orange outline, does concentration effects with tons of colours - turquoise, copper green, etc. Strong "brown fuzzy edge" reaction with the sulfur family. Oh, and it kinda likes to devitrify! Keep it out of the flame or *right in* a hot flame - avoid indirect or weak flame. Varies in colour from a pale pink to deep magenta to deep purple, depending on how it's treated in the flame.
Mosaic green - strong brown-fuzzy edge reaction with sulfur family. Strong concentration effects with cool colours. Spreads out like crazy on non-allied colours (Opal yellow, white). Repels silver.
Intense Black - Stays black when stretched thin. When super-heated, tends to spread out into a Black Lace effect. Bubbles if re-heated after being marvered by a cold tool. Heat tools before marvering to avoid this.
Ruby Gold - Transparent, striking. Doesn't really like sulfur colours. Pretty orange halo on opal yellow. Condenses many copper colours. Goes nuts with silver (greenish/blue galactic/faux boro effect!). Reduces.
There are LOTS more - the mad scientist approach is very helpful! One fun way to play is to mix some fairly non-allied colours and/or colours from *one* of the reactive families (I'd start with the cool copper family). You can heat these combinations pretty hard-core and it won't go brown on you, but you might get fun spreading, mixing and concentrating. When you have finished your base shaping and are happy with your background mix, then add something from the opposing family (ie, sulfur if you started with copper). Heat lightly so that you get the meltage you want and a crisp black outline (which will fade out where it hits the non-copper-family colours), but not so much that you get the brown halo. Sulfur/copper twisties are fun, too!
Hours of entertainment! Enjoy! -Heather