Do I need to paint my base boards?

I would recommend painting your base board for two reasons:

The first being it will seal the base board and prevent glue from soaking in to the wood, which will allow for your grass scatter etc to be more firmly fixed.

The second is that if you are adding scatter to the base board any areas that may be missed will not stand out so much and even give a slight contrast in colour I use a household emulsion with a colour that is close to the colour of the scatter I am using.

Alternatively you could use a brown colour emulsion which would give the appearance of mud under the scatter which also looks affective.

For small areas I would suggest the little tester pots that you can get from your local DIY outlet these have a surprisingly good coverage. For larger areas get yourself a small tub of emulsion or a few more tester pots.