When you are writing your proposal, attention to detail is very important. You must write everything in clear and concise English. You should never use slang and only use the technical terms that you must use. The person who is reading your proposal may be a lay person and become confused if you use too many technical terms or job related slang words and phrases. Color for your graphs and charts is quite acceptable and as it eye catching can mean the difference between submitting a wining proposal and one that ended up in the "also ran" file.
When new freelancers are creating a business proposal template that they are going to use often, they usually miss the most important pieces that make the difference between a good proposal and a proposal that sells. By emulating the sales process, you can create a proposal that solidifies the relationship that you've built with a prospect, and reinforces the close of your sale.