A diamond should be colorless. Colorless here can be considered as white, and the more colorful a diamond is, the more yellow it is.
Here are the diamond ranges of color :
D to F : Colorless
G to J : Near colorless
K to M : Faint yellow
N to R : Very light yellow
S to Z : Light yellow
One cart is equalto 0.2 grams, or 200 milligrams. The value of a 5 carat diamond is much higher than 5 diamonds of 1 carat of the same quality. If the weight is the same, the rarity of large diamonds make the value of diamonds increase exponentially according to their size. This is why carat is considered as a quality criteria, because of rarity.
Choosing the right carat weight will obviously influence on the other criterias of the diamond. Finding an 5 carat, ideal cut, D color, F clarity diamond would be extremely difficult. But you could find these properties in a diamond under 1 carat.
As we say in our build your own engagement ring guide, the first step is to establish a budget. Of course we would all want to offer the perfect diamond ring, but we may also think it's too small, and taking a bigger one would not fit our budget or would not be as clear, colorless or ideally cut.
The choice will remain yours, but here is our recommendation. We of course think the more light a diamond returns the better, not only for grading a diamond but also for the look. A very sparkling diamond produces an exceptional effect.
Cut : at least very good (premium), most of the light is returned
Clarity : at least VS1-VS2, excellent quality.
Color: at least I, near colorless.
Don't hesitate to visit James Allen and Mondera for more education. These are the two stores we recommend to build your own engagement ring. They have interesting features such as look how big a diamond looks, how clear, etc.
To be complete with our diamond education, here are the 10 different shapes you can set on your engagement ring at Mondera and JamesAllen.
The 4 C's :
Color - Carat
You can try the interactive functions at James Allen and Mondera to see the color and size of diamonds.
Similar tools are available on both of stores, in their education sections.

The color grade tool at Mondera.

The carat weight tool at JamesAllen let you see how big a each diamond carat weight looks on the finger. Choose from 1/4 carat to 5 carats.