Only materials/colours created by the LEGO Group are available in the workshop. Fabric materials are a complex topic and have not been fully implemented yet.

I understand. Sorry.

Is Earth Blue the closest we get?

Can you add tr. flour. red & cool silver?

They are in the Workshop, in the transparent section and in the pearlescent section.

Well, they're not there. Even if I click legacy, they don't appear.

They are there:
Transparent section: Tr. Fluor. Red Orange 47
Cool Silver depends on your exact color. What is the LEGO name?

Lego Digital Designer describes it as;
298- Cool silver, drum Icaq. There's also 197- Silver Flip/Flop that I need. The Cool Silver is speckled, but I need non speckled.

298 Cool Silver is Silver Ink in the Metal section.
Silver Flip/Flop doesn't appear to be available in Mecabricks. You could use 131 Silver or 315 Silver Metallic.

http://www.mecabricks.com/docs/colour_chart.pdf

Here you'll find all colours used in Mecabricks with different names in LDD etc to compare.