What minimum number of colours is sufficient to colour all positive real numbers so that every two numbers whose ratio is 4 or 8 have different colours?
1. **Understanding the Problem:**
   We need to color all positive real numbers such that any two numbers whose ratio is 4 or 8 have different colors. This means if \( a \) and \( b \) are two numbers such that \( \frac{a}{b} = 4 \) or \( \frac{a}{b} = 8 \), then \( a \) and \( b \) must have different colors.

2. **Observing the Ratios:**
   Note that \( 4^3 = 8^2 \). This implies that the powers of 4 and 8 are related in a way that can be exploited to find a coloring scheme.

3. **Defining the Function:**
   Define a function \( f(a) = \lfloor \log_2 a \rfloor \). This function maps a positive real number \( a \) to the greatest integer less than or equal to \( \log_2 a \).

4. **Coloring Scheme:**
   We will use the function \( f(a) \) to determine the color of \( a \). Specifically, we will use the value of \( f(a) \mod 5 \) to assign colors:
   - If \( f(a) \equiv 0 \pmod{5} \) or \( f(a) \equiv 1 \pmod{5} \), color \( a \) Red.
   - If \( f(a) \equiv 2 \pmod{5} \) or \( f(a) \equiv 3 \pmod{5} \), color \( a \) Green.
   - If \( f(a) \equiv 4 \pmod{5} \), color \( a \) Blue.

5. **Verification:**
   - Consider two numbers \( a \) and \( b \) such that \( \frac{a}{b} = 4 \). Then \( \log_2 a = \log_2 b + 2 \). Hence, \( f(a) = \lfloor \log_2 b + 2 \rfloor \).
     - If \( f(b) \equiv 0 \pmod{5} \), then \( f(a) \equiv 2 \pmod{5} \).
     - If \( f(b) \equiv 1 \pmod{5} \), then \( f(a) \equiv 3 \pmod{5} \).
     - If \( f(b) \equiv 2 \pmod{5} \), then \( f(a) \equiv 4 \pmod{5} \).
     - If \( f(b) \equiv 3 \pmod{5} \), then \( f(a) \equiv 0 \pmod{5} \).
     - If \( f(b) \equiv 4 \pmod{5} \), then \( f(a) \equiv 1 \pmod{5} \).
   - Consider two numbers \( a \) and \( b \) such that \( \frac{a}{b} = 8 \). Then \( \log_2 a = \log_2 b + 3 \). Hence, \( f(a) = \lfloor \log_2 b + 3 \rfloor \).
     - If \( f(b) \equiv 0 \pmod{5} \), then \( f(a) \equiv 3 \pmod{5} \).
     - If \( f(b) \equiv 1 \pmod{5} \), then \( f(a) \equiv 4 \pmod{5} \).
     - If \( f(b) \equiv 2 \pmod{5} \), then \( f(a) \equiv 0 \pmod{5} \).
     - If \( f(b) \equiv 3 \pmod{5} \), then \( f(a) \equiv 1 \pmod{5} \).
     - If \( f(b) \equiv 4 \pmod{5} \), then \( f(a) \equiv 2 \pmod{5} \).

   In both cases, \( a \) and \( b \) will have different colors according to our coloring scheme.

Conclusion:
The minimum number of colors required to color all positive real numbers such that any two numbers whose ratio is 4 or 8 have different colors is 3.

The final answer is \(\boxed{3}\).