When you have a bespoke template made for your NewZapp account, a range of branded colours will usually have been set within your Custom Blocks.
If you are using one of the free Basic or Themed templates then Black will probably have been set as the default.
No matter which type of template you use, If you need to change the colour of text in your email (including linked text), you can do so if you have the hex value of the colour.
A hex value is a unique string of 6 characters which pin points a colour used for the web / emails.
For example, the hex value for Black is #000000, White is #FFFFFF, and 50% Black (mid Grey) is #7F7F7F.
Within your organisation's branding guidelines you will usually find the hex value for your brand, alongside print references such as Pantone or RGB values.
This animated sequence shows you how to ignore the standard range of colours and instead change the colour of text by inserting a hex value: