Planetary Imaging with 114mm/4.5" f/8 Reflector - Possible?

Short answer - give it a go! I have a small 4” refractor and whilst it doesn’t produce the incredible images you get from large scopes, it nonetheless gives pretty satisfying pictures. It’s a good opportunity to learn about stacking, lucky imaging, sharpening etc. 

To answer your specific questions:

- A 2x Barlow would be helpful. The usual rule of thumb is to image at a focal ratio about 5x your camera pixel size in microns (3.75 um for the ASI120MM). Currently you’re at about F/7.8 and want to be targeting 5 x 3.75 = F/18.75. A 2x Barlow will get you to F/15.6 which is close enough. 
- To produce colour images with your camera you’d need a set of RGB filters (31 mm diameter would be fine) and a filter wheel. You then collect 1-2 min videos using each filter in turn, stack and sharpen and combine to make a colour image. There’s a bit of a learning curve here and you’d also need to use Winjupos to manage the derogation and combination. You’d probably be up for a similar cost to buying a cheap colour camera and the learning curve would be less - might be the easier way to go. 

The Cloudy Nights site has a couple of long-running threads on ‘small bore’ planetary imaging using scopes of 6” or less that will give you a good idea of what you might expect to achieve. The Jupiter thread is https://www.cloudynights.com/topic/398608-small-bore-challenge-jupiter-w-6-or-less