2 things.
1) Its in the stuti shlokas
2) As you want them to be.

Eg Ganesha. As described in Atharvashirsha

He is
रक्तं लंबोदरं शूर्पकर्णकं रक्तवाससम्‌॥
रक्तगंधानुलिप्तांगं रक्तपुष्पैः सुपूजितम्‌॥
(1st image)

His Form is having a Beautiful Reddish Glow (Raktam), with a Large Belly (Lambodara) and with Large Ears like Fans (Shurpa Karna);
He is wearing Red Garments (Rakta Vasam),
His Form is annointed with Red Fragrant Paste (Rakta Gandha),
and He is worshipped with Red Flowers (Rakta Pushpa),

But people also use light colors like skin color etc.  (2nd Image)

Eg 2:
Saraswati (Stotra: Ya kundendu tushar)

या कुन्देन्दुतुषारहारधवला या शुभ्रवस्त्रावृता
या वीणावरदण्डमण्डितकरा या श्वेतपद्मासना ।
या ब्रह्माच्युतशंकरप्रभृतिभिर्देवैः सदा पूजिता
सा मां पातु सरस्वति भगवती निःशेषजाड्यापहा ॥१॥

Translation
Who is Pure White like Jasmine, with the Coolness of Moon,
Brightness of Snow and Shine like the Garland of Pearls; and
Who is Covered with Pure White Garments,
Whose Hands are Adorned with Veena (a stringed musical instrument) and the Boon-Giving Staff;
And Who is Seated on Pure White Lotus,

Eg 3:
Surya (Sun)

लोहितं रथमारूढं सर्वलोकपितामहम् ।

Translation
You are Reddish in colour, and mounted on a Chariot; You are the Grandfather of all persons.

Eg 4. Kalabhairaba (Stotra: Kalabhairav ashtak)

श्यामकायमादिदेवमक्षरं निरामयम् ।

Translation:
Whose Body is Dark, Who is the Primordial Lord, Who is Imperishable and Who is beyond Diseases [of the World]

Eg 5:  Krishna (Stotra: Sri Bala Mukundashtakam)

इन्दीवरश्यामलकोमलाङ्गं
Translation:
Who is like a Dark-Blue Lotus with Tender and Soft Body (Image 1)

But there are idols where krishna is fair. Its as per people prefer.