Note that $ 4^{3}=8^{2}$, We need at least 3 colours. For positive real $ a$, $ f(a)=[\log_{2}a]$, $ f(0)=0$, $ f(a)=f(-a)$. $ f(a)\equiv 0,1 \pmod 5 \Rightarrow$ Red $ f(a)\equiv 2,3 \pmod 5 \Rightarrow$ Green $ f(a)\equiv 4 \pmod 5 \Rightarrow$ Blue Then we are done. The answer is 3.