If it’s going to be hot then singlet is definitely the way to go however still bring the light blouse because you can just throw that in your bag if you get hot. When it comes to colour I say Grey - that’s personal preference of course! I have pulled a look for you here using a grey singlet, demin shorts and a range of brown accessories and sandals. I wouldn’t opt for boots because if the weather is heating up best to leave boots for fall and winter.
Hope this helps