What adjective would you use for "not readable color"?

This isn't exactly what you're looking for, but it will help get the idea across.

The mail has poor contrast

differences in colour or in light and dark, used in photographs and paintings to create a special effect

...and therefore is illegible.

difficult or impossible to read

_[Oxford]

As for your requirement, the best I could come up with is:

the mail is written in eye-popping colors,

but that wouldn't be very apt here, as eye-popping is generally used in a positive sense, and not always in context of colors.

Probably, indecipherable. At the heart of this adjective is cipher, which means "code".

Something that's indecipherable can't be understood. If you can't figure out the meaning of something, it's indecipherable. (vocabulary.com)

• an indecipherable message or font.

Readable color combination might be a suitable phrase.

When two similar matching or saturated colors are combined on a web page, you could call the final result: unreadable color combination

the most readable color combination is black text on white background; overall, there is a stronger preference for any combination containing black. The two least readable combinations were red on green and fuchsia on blue. [...] Also, in every color combination surveyed, the darker text on a lighter background was rated more readable than its inverse (e.g. blue text on white background ranked higher then white text on blue background).

^{Source: On the readability of inverted color schemes}

The expression contrast, as suggested in the comments and by Tushar Raj, is perhaps the most widespread term where web design and readability is concerned. I might describe white text on lime-green background as having low contrast. From the same article:

In any event, the white on blue contrast provides especially good contrast in low brightness environments. And, as William told me, this was very useful when refresh rates were slower (i.e., less than 60 Hz). The white on blue contrast in such situations reduced perceived flicker [...]
Very high contrast is absolutely a readability problem: but white on black is just as high a contrast as black on white.

One web designer appears to have some strong feelings on the subject. In fact the webpage is called contrast rebellion :)

Low-contrast font color and unreadable texts?
To hell with them!

Because a website's content is primarily there to be read.
Don't give your visitors a headache only because gray or any other low-contrast font color looked better on the design comps than black.

Let's put an end to this low-contrast, light gray nonsense and use typography for its purpose: MAKING TEXT READABLE.

^{P.S. It's actually a very eye-catching and well-designed page by Zoltán Gócza and written by Richard Gazdik}