For me chosing the Colour of Bhagavan Shree Vishnu for Hridaya-Dhyana varies upon my mood, but I am more in mad love with Shyama Vishnu for sure.
Brilliant Blue Bhagavan Shree Narayana is Very Beautiful too, don't hesitate to find Ashraya in such Hridaya-Dhyana too.
Futher is the Tika of OM VishnuPad Madhwa Acharya to "Shree Chandogya-Upanishad", (Adhyaya 8, Khanda 13, Mantra 01) cited from Chandra Vasu 1909 translation).
श्यामाच्छबलं प्रपद्ये शबलाच्छ्यामं प्रपद्येऽश्व
इव रोमाणि विधूय पापं चन्द्र इव राहोर्मुखात्प्रमुच्य
धूत्वा शरीरमकृतं कृतात्मा
ब्रह्मलोकमभिसंभवामीत्यभिसंभवामीति ॥ ८.१३.१॥
shyAmAchChabalaM prapadye shabalAchChyAmaM prapadye.ashva
iva romANi vidhUya pApaM chandra iva rAhormukhAtpramuchya
dhUtvA sharIramakR^itaM kR^itAtmA
Translation: "I take refuge with the Lord having all colours through meditation on Him as having no colour : I take refuge in the Lord having no colour by meditating on Him as having all colour. Shaking off all evil as a horse shakes his hairs or as the moon frees herself from the mouth of Rahu, shaking off the body, I obtain the uncreated realm of Brahman, having obtained the Self".
Tika by OM VishnuPad Madhwa Acharya: "In Khanda 13th is taught the method of worshipping Bhagavan Shree Vishnu in His different Colours as Residing in the heart.
One colour of Bhagavan Shree Vishnu as residing in the heart is Dark called Shyama (श्याम). It may be violet colour or pure black.
The colour of Bhagavan Shree Vishnu as dwelling in the heart is also Shabala (शबल) or variegated. The commentary mentions this:
"The colour of Visnu as dwelling in the heart is Dark (Shyama ) as well as variegated (Shabala ) or having various and manifold colours. (Shabala also means a collection of all colours.)
These two forms of Bhagavan Shree Vishnu, One Dark and One White (for white is an aggregate of all colours, and Dark is absence of all colours) both dwell in the heart.
A man must worship and meditate on these Forms of Bhagavan Shree Vishnu.
But there is no order in which ho may meditate on these two forms.
He may begin with Shyama meditation and end with Shabala, or begin with Shabala and end with Shyama.
The order is not crucial".