I just did a presentation on them to the garden club. The white ones can only go greenish and white, the ones that are colors like pink and blue will change colors, the pH and the aluminum content in the soil affects the color ranges. There's a lot of great information if you use Google. They collect aluminum and aluminum is released from acidic soil, it forms complexes in the hydrangea flowers giving them their blue color. So for blue add aluminum sulfate around the base soil. Adding coffee grounds will also lower the pH. For pink blooms you don't want aluminum, add dolomitic lime several times a year this will raise the pH shoot for a ph of 6-6.2. Since they take up aluminum best at lower ph, raising the ph will help keep the blueing effect of aluminum out of the hydrangeas system.
Phosphorus helps to prevent aluminum from creeping into the system of hydrangea.

This is info from my presentation that might be useful. I have a ton so feel free to ask!

Growing Hydrangeas
Propagation of Hydrangeas is rather easy with cuttings obtained from the ends of non-flowering shoots with two or three pairs of leaves, from April to August.
Hydrangeas are easy to grow in well-drained soil, which should contain plenty of organic matter or humus.
It is best to root them in sand in a shaded area. Avoid planting it in hot, dry, exposed sites.
For planting, dig a large hole approximately two feet across and one foot deep.
Hydrangea plants grow without difficulty in a wide variety of soils but prefer fairly rich moist soil.
A general-purpose fertilizer, such as 10-10-10 applied at a rate of 2 cups per 100 square feet in March, May and July is suggested.
It is not necessary to remove the mulch when fertilizing, but water soon after application to help dissolve the fertilizer and send it into the soil.
Hydrangeas can grow in full sun if they are watered well, but will bloom more freely in partial shade.
Hydrangeas need plenty of water, so plan to water thoroughly once per week or more frequently.
Pruning Since the flowers appear on previous year's growth, prune only the stems that produced this year's flowers; otherwise you will not have blooms next year.
it's important to know that mophead hydrangeas do not have to be pruned back - ever - unless they are very old.
Plant in early summer or fall. Don't plant a hydrangea during the hottest part of the summer unless it can't be helped.
Transplant a hydrangea when it has become dormant and has lost all of its leaves (late fall or winter).
All hydrangeas will bloom and grow well in morning sun and afternoon shade.
No hydrangea will do well in HEAVY shade, such as under a shade tree. The blooms will be sparse and will not develop fully.
If your landscape is mostly sunny (and hot), you may wish to grow the PeeGee (paniculata) hydrangea, which thrives in all day sun as long as it receives adequate moisture. PeeGee hydrangeas actually need at least 5 hours of sun per day to bloom well.
The Oakleaf hydrangeas will also grow in sun or shade, but the blooms last longer if they get a little afternoon shade in hot climates. The leaves on the Oakleaf hydrangeas will "color up" best in the fall if they receive some sun.

ypes of Hydrangeas
There are 3 types of flower blooms in Hydrangea. They are -
Mophead - Globe shaped flower cluster, the most commonly recognized form of Hydrangea bloom.
Panicle - Long, somewhat cone-shaped flower cluster (particularly in Oakleaf Hydrangeas).
Lacecap - Flattened cluster of what appear to be tiny, immature buds surrounded at the edges by typical 4 to 5 petal flowers.

Water your new hydrangea consistently during its first spring, summer, and fall. The larger the plant you have planted, the more it will need watering in the first year to compensate for its reduced root system. A thorough drenching of the root system once or twice a week is infinitely preferred to a daily “sprinkle.”
Hydrangeas have a reputation for being “water hungry.” Adequate water is critical in the high growth months of spring. However careful planting and mulching can go a long way to reduce watering needs.
Many hydrangeas will wilt during the heat of a summer afternoon. This is because their large leaves are giving off water faster than the roots of the plant can replace it. As soon as the sun goes down, they perk back up. Don’t assume that a wilted plant needs watering. Check the soil moisture level to a depth of 4-5”. If it is moist, put off watering until the soil is dry.

WHITE HYDRANGEAS can NOT be changed to pink or blue by the grower.

How can I change the color of my hydrangeas.
one of the links I used to find information

Pink/Blue hydrangeas:

These show pink blossoms in basic (alkaline) soils and blue in acidic soils. Most of the soils in Wisconsin are basic due to the over abundance of limestone. There is a product sold in the local lawn and graden stores that is supposed to slant the blossoms to rhe blue hue. It constains a sulphur compound to acidify the soil. I tried this year per instructions but with little effect.

I do have an interesting effect this year. Now, that frost time is near and the nights have cooled, my two plants have suddently pushed out large leaves and have developed several blossom buttons. Last year at this time, they were shutting down. Those new blossoms are gong to get frost bit.

A professional soil test is useful. Most of our state Extension services will do this for very little cost. A soil test will not only tell you what the basic pH is, but other information. We shouldn't load the soil with lime or aluminum without knowing what we're dealing with.

Thanks everyone for all the info. That is great and much appreciated