I use powdered natural oxide pigments made for concrete.
White and green are more expensive, and blue is really expensive but if you use layers and only color the outside 1/2 inch or so you have color that will last forever even when chipped and scrapped.

Ignore the percentages rules and color charts.
The % rules are based on keeping constuction grade concrete safe. I have used 3 times as much as the maximun allowed and still have strong concrete.
Color charts are based on those small % of colorant in GRAY Portland.
If you use white portland you can get brighter colors, even dark cobalt blue.
Using copper slag instead of sand can result in PITCH Black concrete that actually glitters like metalic paint in the sun.
Buy some colorants and experiment!
Good Luck!

Might want to consider costs on your colours.Sticking with the ratios as recommended by the manufacturer is fine.For really bright colours,as stated,use white portland with the colour of your choice.
Besides concrete colours,check with your local pottery store.They have oxides and others in small batches for you to try.
For white,try white portland,silica flour,available from the aforementioned pottery store,and fine perlite.
Again,as mentioned,coloured aggregates will lend their colour to your finished piece.
Play in the mud-it's good,clean fun!!
Cheers from here

White pigment makes a big difference to how white your piece actually stays.
It's interesting that a local gravestone maker finds it cheaper to make white stones using the cheap GRAY Portland with white pigment.
White pigment mixed with other colors modifies them considerably. White pigment is very handy.
Be careful with pottery pigments, they are not all UV and concrete compatible and will fade fast.

If you go to a pottery supply house,the pigments will vary.I chose the oxides,and have had good success with them to date.
As to white,one recipe for white that I've read about includes lime as an additional ingredient.I've not gone there,as I'm not trying to get that bright a colour.We 're trying to get more natural,and the colours to date that we've used get us there.We'll be trying others,like stains,when we start airbrushing.The rock bluffs that surround our home give us daily inspiration for subtle shading,and hopefully you can find such a muse in your area.Good luck with your colour choices.May they bring you satisfaction.
Cheers from here

A year or so ago, I put together a couple of "guides" for using various colorants in cement-based mixes. These guides are now resident on a little site called The Artistic Garden. They cover most of the basic considerations and were meant as a starting point for experimentation. Take a look if you like, that are in a PDF format for easy downloading. Hope they help a bit.

All my best...Tango

Here is a link that might be useful: Cement Colorant Usage Guides as PDF's

Nice site,"The Artistic Garden".Good refs on colour as well.Good on you for putting it together.
Cheers from here

White isn't a natural color?
It is where I live.

Chromium Oxide is a very strong green. The price has skyrocketed since the last time I bought it. It's $12 a pound now. Titanium white should be ok to use - it may even have selfcleaning properties in the right formulation.

The Iron Oxides are known to be extremely stable and may even strengthen the cement. Black IO is very strong. I can't really see the difference between the Red & the Indian Red IO, and the Brown is a very red shade of brown (but the Chromium Oxide green can moderate it). The Yellow IO is more ochre than lemon.

I once bought a small box of wonderful cement blue from Rainbow Colors - a cross between cerulean and cobalt in color. When I went back to the original place, they said that was a special order and I'd have to order a case. I'm going to try the ferrus sulphate, copperas, and see how that turns out.

I haven't found a true red yet.

Here is a link that might be useful: Big Ceramic Store

"Chromium Oxide is a very strong green. The price has skyrocketed since the last time I bought it. It's $12 a pound now."

WOW!
You need a better supplier. An industrial supplier.
I pay less than $25CDN for 7 pounds of the best Chromium Oxide. White is the same price. All others are cheaper. EXCEPT Blue which costs about $8 a pound in 7 pound bags.

Some companies charge $15-20 a pound for blue.