How can I change the back ground colour on my text messages. It is green with white writing and I find it hard to read

Hi. I have an IPhone 6. Model number is MG4J2X/A, IOS 8.4.1. I have difficulty reading the copy of the text messages that I send. The messages that I send have a lime green background with the words written in white. I notice too now, that some text messages I send are in blue? I don't know how come, but at least I don't have difficulty reading the blue. How can I change the colour of the background text messages permanently from the Lime Green colour? I tried Settings/Accessibility/invert colours - but this inverted the colours on everything, photos, wallpaper and everything. So this is not suitable.