You can make any color if you have a Red Monster and a Yellow Monster (or DNA for them).
If you use all of the DNA making other colors, you could lose that Monster. Some Monsters can be remade through combinations, but others are rare and harder to get back. Always make sure you have enough DNA or have that Monster in your inventory or in your area with enough DNA to collect to make more. I try to keep several Red and Yellow Monsters of each type in my inventory so I can place one if I need more DNA. If you do this, remember to replace the eggs you use.
You can also lose a Monster if you only have 2 DNA and the breeding fails. Be sure to save enough gold to protect the DNA incase of this happening.
When Placing Eggs, if you leave the area before the Eggs cracks fully open they will just sit in your area unhatched. Since the timer doesn't start until an Egg hatches, you can lose a lot of time if this happens and you fail to notice.
If you are missing DNA from a Monster in Friendship Areas, click on the plant/totem/shell in the background in your Friendship Area. You should be able to collect any umclaimed DNA here.
Remember that you only get half of the DNA from Monsters in Friendship Areas and that the friend gets the other half.
Some Monsters only give out 2 DNA as you progress through the game. It's easy to think you are missing DNA if you've overlooked this.
Daily Quests are important to get Pearls. You'll need Pearls for Diamond Phials to create Elite Monsters.
Daily Quests usually consist of:
1) Place a certain number of a certain color of Monster in your area.
2) Visit 5 friends (once a new game day begins, do your daily task before visiting friends so that you can complete the task if you only have a few friends.)
3) Help 5 of your friends Monsters. (collect friends DNA 5 times.)
4) Place 3 of any Monster of any color in your friends area.
5) Collect 6 DNA from your Monsters and give 3 green leaves. (You get green leaves from the reward for collecting all 7 of a Monster type.)
You can try to buy green leaves from the Market, but beware of the prices. It's not logical to spend over 333 Pearls for 1 leaf, as that is all you'll get for them from the Daily Quests.
Mining is important to upgrade your Monster Areas to hold more Monsers, and to upgrade your breeding platforms so you can use more Potions and Powders to get a better chance of success.
You have 5 Workers to mine with in your Home Area and your friends Home Areas. You'll want to start mining as soon and as often as you can if you want to keep everything upgraded.
Doing Quests is the best way to get XP and Gold. You get a Quest for new Green, Blue, and Purple Monsters of each Monster type. Completing Multicoloured Monsters gives good amounts of XP also.
Another good way to level up faster is to breed Red Backsacks over and over. This works best if you have at least 2 breeding platforms available, and have plenty of room in your Insect Monster Areas. You get XP each time you collect DNA and Dismiss a Monster. Red Backsacks take the least amount of time to breed and to be ready for DNA collection. It can get boring doing this over and over, but it does show results.
When you breed a Monster type over and over, they will stop producing Gold, and then even XP. This is based on the Monster type, not the color, so you can't simply go from Reds to Yellows and so on. If you use this method often, you'll have to climb up the list of Monsters to keep receiving Gold or XP. This means it'll take longer to breed and raise the Monsters you farm.