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.
In the business world many companies ask more than one supplier to submit a proposal showing what they might charge for a particular product or service. If you are new to the supply side of this situation you may need to refer to a sample proposal template to make sure that the bid you plan to submit is in the correct format. In many cases if a proposal is not formatted properly the company who has requested the bid will not even bother to look at it.