With sexing the blue/opaline/gold etc gouramies (they are all the same species - trichogaster trichopterus), all you need o do is look at the dorsal fin in mature fish and you'll notice that the male's is significantly more pointed than the female's. Males also generaly have longer fins and females are more wide-bodied when viewed from above or more 'rounded'. I could sex them for you if you have pictures of them from the side.
Dwarfs are even easier to sex as the males are colorful while the females are silvery-grey. I'm assuming yours are likely both male as females are rarely sold at most LFS due to their lack of color. Their fins are also quite different but sexing them based on color is easier if you don't already know what to look for with fins.
As for crossing golds and opalines. Of course it can be done, as they are the same species, and you will not get 'mutts' because the different colors will not mix... One color will be more dominant than the other and the majority of fry will likely be that color - not something in between. However, it has been suggested that crossing different color morphs of the three-spot gourami can be difficult because the fish sometimes view each other as different species (who knows why) or the males show a different prefference for color than the females or vice versa (quite complicated but don't worry about this).
If you do have an opaline and gold of opposite sexes however, these are deffinatly the easiest gouramies to begin with. The natural 'blue' color morph is generaly more prolific but these will do
You need to begin by doing some major google searches
Then prepaire yourself for what's to come. Because these are relativly large fish, you WILL need a grow-out tank for the fry and you WILL need to get rid of them as they mature or you will end up with a lot more fish than you or your tanks can handle.
Note: The following also applies to dwarf gouramies.
Then you need to seperate the male and female and condition them with live or frozen, generaly rich foods. I usualy only do this for about a week (with pearl gouramies) but mine are pretty much always in breeding condition anyway. If you find they don't breed the first time you put them in together, re-condition them for longer.
Then you need a 10 gallon tank (no less or you'll have trouble... up to 30 or the fry won't be able to find food). Set the temperature to 80 deg. F and put in a sponge filter from a cycled tank (leave the sponge running in an established tank for a week before introduction). Finaly, introduce the female to the tank followed by the male a little later. Including some floating plants/anything floating
and lowering the water level to about 1/3 the normal depth may help the male start building a bubble nest. Also put in a few fake plants so the female has places to hide once they are done spawning. A tight-fitting lid is essential to keep the temperature above the water consistantly warm but do not include a substrate. Having an airstone ready is advisable.
If all is well, the male will build a bubblenest and then intice the female under it where they will embrace (like bettas) and she will release eggs which he will fertilise. He will then proceed to collect these, retrieving them and placing them in th nest. Once they are done, the male will start chasing the female around and guarding HIS nest. Take the female out at this point - she no longer plays a part in the spawning - and give her a chance to recouperate.
For the next couple of days, the male will guard his nest. Don't bother feeding him as he may not eat and you risk polluting the water. Make sure the tank is in a quiet place and don't interfere or stress the male as he may be tempted to eat his spawn and leave it at that. If necessary, cover the tank with a towel. HOWEVER, if there are no lights in the breeding tank, don't leave him in the dark all day or he will not be able to retrieve any eggs that fall or 'clean' the eggs he is caring for in the nest as he won't be able to see them clearly.
Once the fry are free-swimming, it will become obvious that the male can no longer successfuly keep up with them or guard his nest. Take him out and feed him. He has done his job well.
You'll likely notice now that the fry are TINY. They require TINY foods. This is where prepairing beforehand is priceless.
You will need to feed the fry on infusoria for the first 4-5 days. You can start a culture a week or so before or you can get live plants and some 'liquid food' for egglayers and add those. The liquid food generaly is a food for infusoria rather than directly for fry - that's why you need to add live plants (plants usualy have infusoria). If this is going to be your approach, add the plants BEFORE the male and female so that you are not stressing the male out by doing it while he is still guarding his nest but are still giving the infusoria a chance to multiply.
You should also start feeding microworms as well. Make sure you order a culture (if you haven't got one already) and set it up a week or more before so that you have a healthy number to feed the fry at least 3 days after they are free-swimming. The microworms will survive for almost 24 hours in the water and prove irresistable food for your fry.
You can also feed baby brine shrimp but these don't last as long, are larger than microworms so impossible for newly-hatched fry to eat and may cause swim bladder problems if the shells somehow get eaten. I never use thm and would swear by microworms any day.
For more information on culturing microworms and infusoria etc, I think you can check pinned topics in the betta forum. There may be something on live foods in the tropical chit chat forum as well, or the breeding forum, but I am not certain.
Remember that the first week or so is when the fry are at their most vulnerable stage. Particularly at the end of the first week, they are developing their labyrinth organs and it is essential that the air above the water is warm or they will be shocked and die.
Also, you may want to turn on an airstone (low) at this stage. It'll break the water's surface tension and make it easier for the fry to breathe. You should do this right after you take out the male, in fact, as the extra circulation will do them good.
Remember that clean water is essential and that, after about 2 weeks, you will want to gradualy increase the water level until the tank is completely full - this will also serve to keep the water quality good while the fry are still so small that it is riky to do a water change without sucking up fry (use a white bucket so you can see them and just put them back in the tank...).
Anyway, as they mature, you should begin feeding them on crushed flake and commercial foods so that they learn to eat these and do not prove troublesome to feed as adults. Once they reach about 2 inches in length, it's time to sell them or give them away or whatever you have planned for their futures.
That's pretty much it but as you can see, livebearers are much easier to breed. I actualy reccomend you try breeding zebra danios or convict cichlids before you try gouramies though. The reason being the small foods required and the space the larger species need. Also keep in mind that three-spots (the opaline and gold) gouramies are rather aggressive so watch the pair whilst spawning to make sure the male isn't too bad with the female.
Again - research, research and research. Google.com is incredibly useful for this sort of thing
Anybody fill me in? And secondly, if I have a male and a female and they are different types, could I still try and breed them? (eg - say I had a female golden and a male opaline) or would they have to be the same type. Also, could someone please give me instructions as to how to breed them. I am not really a newbie and have bred livebearers so I do have a bit of experience. 
