A Colorado ballot proposal to raise state income and sales taxes for the sake of public schools was headed for defeat on Tuesday, according to early election returns from the state’s most populous counties, Reuters reports. The outcome of Proposition 103, aimed at generating $2.9 billion over the next five years, was being watched closely as an indication of voter sentiment on tax hikes in a key battleground state for the 2012 presidential election. It could also shape tax-related ballot measures expected to surface in several other states next year…