If you think that it is childish that Jesus has a rainbow coloured horse, you’re also going to be disappointed with His throne. In Ezekiel 1:26 we see that the throne of the Holy Spirit looks like lapis lazuli:
26 Above the vault over their heads was what looked like a throne of lapis lazuli, and high above on the throne was a figure like that of a man.
Ezekiel 1:26 NIV
This is what lapis lazuli looks like:

Personally, I am so glad that our God loves bright colours.