Loading in the custom data all works well if I want to call any of the following in OnData()

yld["Open"] = float(data[1])
yld["High"] = float(data[2])
yld["Low"] = float(data[3])
yld["Close"] = float(data[4])
yld["Adj_Close"] = float(data[5])
yld["Volume"] = float(data[6])

eg data["grid"].Close


but if I try:

yld["blue_neg_one"] = float(data[8])

data["grid"].blue_neg_one I get the following error.

AttributeError : 'Data' object has no attribute 'blue_neg_one'


essentially my question is, How do you access the other column when importing custom data. Having read through the examples there don't seam to be any applicable refrences to this.