If your lawn looks like a patchwork of different colors of green, it probably is a mixture of types or varieties of grasses. If you look closely you will notice they grow at different rates and the leaf shape will be different between the lighter and darker patches of color. The best method of controlling weeds in a lawn is to maintain a dense and vigorously growing turf cover.
If the patches are a broad-leaf weed such as Dandelion or crab grass, then a selective herbicide can work. You can also apply Roundup and kill the crab grass and then reseed these areas.
For mixed types of lawn grasses you can aerate and over seed your lawn.It can be tough to selectively kill mixed varieties of lawn grasses. If they are in isolated in small patches, then kill with Roundup and reseed.
If you lawn is too bad, then remove the lawn with a sod cutter and replace your lawn with premium sod from your local sod farm like Far West Turf and Nursery. Sod for sale in Vancouver WA.