How to calculate Helmert Coding I am trying to understand how Helmert Coding works
I know it compares levels of a variable with the mean of the subsequent levels of the variable, but what are these levels and how can I calculate this mean?
This is the example I am using:

Can someone explain how the cells in yellow are calculated?
 A: With Helmert coding, each level of the variable is compared to "later" levels of the variable.
The weights depend on the number of levels of the variable.
If there are L levels then the first comparison is of level  vs. $(L-1)$ other levels. The weights are then $(L-1)/L$ for the first level and $-1/L$ for each of the other levels. In your case L = 4 so the weights are .75 and -.25 (3 times). 
The next comparison has only $L-1$ levels (the first level is no longer part of the comparisons), so now the weights are $(L-2)/(L-1)$ for the first level and $-1/(L-1)$ for the others (in your case, $2/3$ and -$1/3$.  And so on.
Why are you using Helmert coding here?  As this page notes, Helmert coding and its inverse, difference coding, really only make sense when the variable is ordinal.

Clearly, this coding system does not make much sense with our example
  of race because it is a nominal variable.  However, this system is
  useful when the levels of the categorical variable are ordered in a
  meaningful way.  For example, if we had a categorical variable in
  which work-related stress was coded as low, medium or high, then
  comparing the means of the previous levels of the variable would make
  more sense.

Personally, I find them hard to interpret, even in that case. But, you are comparing "White" to the average of the other three groups. Is that what you want?
