Headphones will have to do for now, until I can afford decent monitors
Ahh - I've been there! I worked for a long time with a pair of Hi-Fi speakers and a pair of Sennheiser HD250 headphones. Mixing with the headphones gave me a dark mix, cos they were so bright and detailed, and mixing with the speakers gave me an overly bright mix. Eventually I learned what to listen for and how to compensate for each.
Given enough experience with any setup, you can learn how to adjust for it when you mix. Just try your mixes everywhere - especially on a car CD player - until you learn how to compensate.