I think your hunch that you have a soil problem is right, but I wouldn't assume that it's an acidity issue. It's likely a fertility issue. Verify this by testing your soil. Soil test results will let you know if the soil is too acid (or too alkaline) and which nutrients, if any, might be lacking. If your soil is too acid, apply lime at the amount recommended by the test. If it's a fertility issue, your lawn will benefit from fertilizer. Apply a turf fertilizer, and keep an eye on the grass to see whether it changes color. If so, you've found the problem, and you should begin a regular fertilizer program. Soil on new construction sites may be a mix of topsoil and lower-fertility subsoil. Lighter green patches could be due to low fertility in those spots. Fertilizing the entire lawn will mask the differences.