In this colour-based odd-man-out question from verbal aptitude, you are given four colour names: Cyan, Crimson, Indigo and Sky blue. Three of them are single-word basic colour names, whereas one is a descriptive two-word shade name referring to the colour of an object. Which option is the odd one out on this basis?

Introduction / Context:
This question tests verbal reasoning through colour vocabulary in an odd-man-out format. The options are Cyan, Crimson, Indigo and Sky blue. In competitive exams, such questions often rely on your ability to classify words by type, such as basic colours, shades or descriptive phrases. Here, three words are single, basic colour names, and one is a descriptive two-word colour term linked to an object in nature.

Given Data / Assumptions:
The four options are Cyan, Crimson, Indigo and Sky blue.
Cyan is a recognized colour term used in design and printing.
Crimson is a deep red colour name.
Indigo is a deep blue-violet colour from the spectrum.
Sky blue is a descriptive term that refers to the colour of the sky, and is written as two words.

Concept / Approach:
The main idea is to classify the given terms into basic colour names versus descriptive phrases. Cyan, Crimson and Indigo are single-word colour names commonly treated as independent colours. Sky blue, however, is not just a simple name; it is a two-word phrase that describes a particular shade of blue inspired by the appearance of the sky. This structural and semantic difference makes Sky blue the odd one out, even though all four are related to colours.

Step-by-Step Solution:
Step 1: Examine Cyan. It is a single-word colour name, commonly used in printing (as part of the CMYK model). Step 2: Examine Crimson. This is another single-word name representing a strong red shade. Step 3: Examine Indigo. It is also a single-word colour term, often mentioned as a spectral colour between blue and violet. Step 4: Examine Sky blue. This is a two-word phrase and specifically refers to the colour of a clear daytime sky. Step 5: Conclude that three options are single-word basic colour names, while Sky blue is a descriptive two-word shade name, making it the odd one out.
Verification / Alternative check:
You can verify by thinking of how these words appear in a typical list of colour names. Cyan, Crimson and Indigo usually appear as independent items in colour charts. Sky blue, however, is often listed as a variant or shade of blue, not as a fundamental colour category. This supports the idea that it belongs to a different subcategory. Another quick check is simply to count the words: Cyan, Crimson and Indigo each have one word, whereas Sky blue has two words, which immediately signals a structural difference.

Why Other Options Are Wrong:
Cyan is a single-word basic colour name and therefore belongs with Crimson and Indigo rather than being the odd one.
Crimson is also a single-word colour term and fits neatly with the group of simple colour names.
Indigo is another single-word colour and stands in the same category as Cyan and Crimson.
Sky blue is distinct because it is a descriptive, two-word name representing a specific shade linked to an object (the sky).

Common Pitfalls:
A common mistake is to focus only on whether the colours are warm or cool, which does not yield a consistent grouping here. Some candidates may also think about which colours belong to the rainbow, but the set is not constructed strictly from spectral colours. The intended logic is simpler: word structure and basic versus descriptive naming. In odd-man-out questions, it is important not to overcomplicate and instead look for the most straightforward property that groups three items together.

Final Answer:
The colour term that is structurally different as a descriptive two-word shade name is Sky blue.