Why are umbrellas black? Why are umbrellas specifically in black? Of course we do have colored ones, but black is the majority. Is there any scientific reason behind it?
 A: Physics answer
A black body at a temperature T, area A (on either side) will radiate heat by:
$$\dot{Q}_{radiated ,downwards}=\sigma A T^4$$. If incident radiations are of power $P$, then $$\dot{Q}_{body}=mc\frac{dT}{dt}=P-2\sigma AT^4$$ (the 2 comes into place because it radiates in both directions)
We can integrate this if we want, but we mainly want to qualitatively analyse it. From this equation, we can see that as time passes, temperature will increase to an equilibrium value ($T_{eq}=(P/2\sigma A)^{1/4}$). I am assuming that $P>2\sigma A T^4$ in this analysis (I've never seen an umbrella that feels hotter than direct sunlight). At this equilibrium value, you will receive heat $\sigma A T^4$, i.e., $\frac{P}{2}$.
For a non-black umbrella, assuming no transmitted heat, we have $e=a=1-r$ (coefficient of emission,absorption and reflection respectively). Out of incident heat power $P$, $aP$ is absorbed. The body radiates heat on either face as $e\sigma AT^4=a\sigma AT^4$. So we have $$\dot{Q}_{body}=mc\frac{dT}{dt}=aP-2a\sigma AT^4$$. From this, we get the same value of equilibrium temperature as for a black body. But, radiated heat power is $eA\sigma T^4$, so it will radiate less heat than the black umbrella onto you (Since both $T=T_{eq}$ are the same).
So paradoxically, we get that a black umbrella is the worst thing to use on a sunny day. 
IMHO
Black seems to be the majority because of a cultural bias. I'm assuming you're from India (by looking at your name). Since India was once a British territory, we can trace the use of black umbrellas back to the Brits. The Brits used to use black because black was considered formal attire (Black suits are actually the most uncomfortable from the physics point of view). This must have proliferated to India (Most British traditions have). Umbrellas before the British must've been colorful (See the painting here). If you take a look at Japanese umbrellas, they're all colorful. So black is really not the majority worldwide.
