Question

How red and yellow soils are formed?

Answer 1

Red and yellow soil are just a same thing with two names.

They are in situ soils formed by the decomposition of old rocks.

Answer 2

Red soil is a type of soil that develops in a warm, temperate, moist climate under deciduous or mixed forest, having thin organic and organic-mineral layers overlying a yellowish-brown leached layer resting on an illuvial red layer. Red soils are generally derived from crystalline rock

Yellow soil is formed under broad-leaved forests in humid subtropical regions, chiefly on parent material from clayey shales. It has an acid reaction and low humus content, and its yellow color is caused by the presence of ferric hydroxide.

Only difference is that yellow soil contains less FERRIC HYDROXIDE than red soil so it is yellow.