Plants taking over my lawn
Thanks for your question. It looks like you've got a few different weeds in the photo you sent. It looks like some kochia and some bindweed. Both of these weeds come in when turf is insufficiently watered and fertilized to out compete weeds. Is your lawn on a sprinkler system? If so you could perform a catch-can test to determine if you have "skips" in the watering system. Place containers of the same size throughout the lawn and run your system. Look to see how much water is going down. A bluegrass lawn needs 1-2" of water a week usually spread over several watering cycles.
Secondly, how often do you fertilize the lawn? Generally lawns need between 3 and 4 lbs of nitrogen per 1000 square feet each season, applied in spring, summer and fall. See http://extension.colostate.edu/docs/pubs/garden/07202.pdf for details on application times and rates.
If the entire lawn looks like this your best bet may be to overseed with new grass seed, this is a great time of year to do it. Sod is also an option albeit much more expensive. To overseed your lawn aerate the soil very well, pull plugs with around 2" between plugs. You can seed into the holes left as they make great germination chambers. See our fact sheet on lawn renovation for care of the new seed: http://extension.colostate.edu/topic-areas/yard-garden/renovating-the-home-lawn-7-241/.
I hope this is helpful. We also do offer lawn check site visits for $50 if you are interested in an in-person look and examination.
Good luck and please let me know if you have further questions.