I haver downloaded the trial, is there a way to change the colors in the workspace like to can in 1.5?
Yes, but it's a bit different. Go to the Edit>Preferences>Colours tab, and you can change the look of things there, although there are less options in this version - you can't change the tool colours, for instance. The spectral colour controls are separate now - Window>Spectral Controls.
A lot of people turn the overall UI brightness down - it's a lot easier on the eyes, and also you can change the overall tint of it, although I'm quite happy with the grey myself.
I do mostly liners. sweepers and promos. Is there really a need to upgrade?
If I'm brutally honest, then 'no', if you are getting the results you want with 1.5 - which a lot of people leave on their systems anyway. The biggest differences now over 1.5 are that recording in MV is direct-to-disk, and that the crossfades work - rather well, actually. The one thing that might benefit you is the multiband compressor - you might well find a use for that. Also you need a soundcard that has a sensible ASIO driver (nothing at all from Creative Labs, for instance), otherwise you'd have to use something like ASIO4ALL to get the best from your existing one with a conversion from ASIO-only Audition to WDM. And if for some reason you want to use more than one sound device, you'd have to use ASIO4ALL anyway, because ASIO as a standard only supports single devices. So it's a good job that ASIO4ALL is free!
But people are turning out good work with 1.5 - it's probably the most stable version yet, one way or another. It may well be worth staying in the upgrade path though, because it will cost you less in the long run, and there's nothing that says you have to lose any earlier versions - they all co-exist quite happily on one machine.