API 1 - currencylayer
currencylayer provides current rates and conversions of many different types of currency. The currency conversion updates hourly, you can use historical rates, get basic support and HTTPS encryption. It has a free version that gives daily updates, historical rates, but limited support and is limited in requests. The basic kit is $9.99/month. Enterprise is $39.99 and updates conversions every 10 minutes. The most expensive is Enterprise+ at $79.99/month and updates every 60 seconds and includes higher quality support and more features.
API 2 - xColors
xColors can generate random colors, including specific random colors such as wanting a random blue. xColors is completely free to use.
API 3 - nager.date
nager.date allows you to view upcoming holidays around the world. It is open source and free to use for personal and commercial.
Summary
currencylayer is a free and paid upgrade service that allows you to get currency rates and conversions. xColors allows you to get random colors added to your site, including random specific colors, such as you choosing blue. xColors is free to use. nager.date gets you upcoming holidays from around the world and is free to use.